I think this is just a case of ambiguity, or polysemy: dürfen usually expresses permission and nicht dürfen a probihition. The latter is possible because the negation has scope over the modal, as expressed by the paraphrase in quotation marks.

Er darf keinen Alkohol trinken.
"Es ist nicht der Fall, dass er Alkohol trinken darf."

Compare this to the English translation, where the modal has scope over the negation.

He must not (is not allowed to) drink alcohol.
"It is necessary that he not drink alcohol."

Note that because English modal verbs are defective, only be allowed to can be used in the past. Two examples with Indikativ Präsensperfekt ("Perfekt") and Konjunktiv Präteritumperfekt ("Konjunktiv II der Vergangenheit"; for the IDS terminology, see Konjunktiv und Tempus):

Er hat keinen Alkohol trinken dürfen.
He was not allowed to drink alcohol.

Hätte ich doch nur Alkohol trinken dürfen!
If only I had been allowed to drink alcohol!

However, the difficulty in this case does not arise from the issue of the scope of the negation (nicht dürfen versus must not), nor from the subjunctive. Rather, dürfen plus negation is used here to express a suggestion or a request (see Duden 1b and DWDS 1c).

Du darfst ihn nicht so ernst nehmen.
Don't take him too seriously.

So darfst du nicht denken!
Don't think like that!

The original sentence uses Konjunktiv Präteritumperfekt, i.e. the perfect with the auxiliary in the Konjunktiv II. It is not about permission, but about a suggestion that has been ignored: It would have been a good idea for the boy not to speak to them, but he did in fact speak to them.

Der Junge hätte nicht mit uns reden dürfen, das ist gegen das Gesetz.
The boy should not have spoken to us, it's against the law.

Of course, the sentence in itself is perfectly ambiguous and could also be used to express that a permission would not have existed, if not for a certain condition.

Der Junge hätte nicht mit uns reden dürfen, wenn er nicht der Sohn des Häuptlings wäre.
The boy would not have been allowed to talk to us if he wasn't the chieftain's son.

I think this is just a case of ambiguity, or polysemy: dürfen usually expresses permission and nicht dürfen a probihition. The latter is possible because the negation has scope over the modal, as expressed by the paraphrase in quotation marks.

Er darf keinen Alkohol trinken.
"Es ist nicht der Fall, dass er Alkohol trinken darf."

Compare this to the English translation, where the modal has scope over the negation.

He must not (is not allowed to) drink alcohol.
"It is necessary that he not drink alcohol."

Note that because English modal verbs are defective, only be allowed to can be used in the past. Two examples with Indikativ Präsensperfekt ("Perfekt") and Konjunktiv Präteritumperfekt ("Konjunktiv II der Vergangenheit"; for the IDS terminology, see Konjunktiv und Tempus):

Er hat keinen Alkohol trinken dürfen.
He was not allowed to drink alcohol.

Hätte ich doch nur Alkohol trinken dürfen!
If only I had been allowed to drink alcohol!

However, the difficulty in this case does not arise from the issue of the scope of the negation (nicht dürfen versus must not), nor from the subjunctive. Rather, dürfen plus negation is used here to express a suggestion or a request (see Duden 1b and DWDS 1c).

Du darfst ihn nicht so ernst nehmen.
Don't take him too seriously.

So darfst du nicht denken!
Don't think like that!

The original sentence uses Konjunktiv Präteritumperfekt, i.e. the perfect with the auxiliary in the Konjunktiv II. It is not about permission, but about a suggestion that has been ignored: It would have been a good idea for the boy not to speak to them, but he did in fact speak to them.

Der Junge hätte nicht mit uns reden dürfen, das ist gegen das Gesetz.
The boy should not have spoken to us, it's against the law.

Of course, the sentence in itself is perfectly ambiguous and could also be used to express that a permission would not have existed, if not for a certain condition.

Der Junge hätte nicht mit uns reden dürfen, wenn er nicht der Sohn des Häuptlings wäre.
The boy would not have been allowed to talk to us if he wasn't the chieftain's son.

Two further examples similar to the original (although I am not so sure about the second, which seems almost like an idiom).

Ich hätte nicht so viel essen dürfen, jetzt habe ich wieder Sodbrennen.
I shouldn't have eaten so much, now I have heartburn again.

(Wie war der Urlaub am Meer?) – Das Wasser hätte wärmer sein dürfen.
It would have been nice if the water had been warmer.

I think this is just a case of ambiguity, or polysemy: dürfen usually expresses permission and nicht dürfen a probihition. The latter is possible because the negation has scope over the modal, as expressed by the paraphrase in quotation marks.

Er darf keinen Alkohol trinken.
"Es ist nicht der Fall, dass er Alkohol trinken darf."

Compare this to the English translation, where the modal has scope over the negation.

He must not (is not allowed to) drink alcohol.
"It is required that he not drink alcohol."

Note that because English modal verbs are defective, only be allowed to can be used in the past. Two examples with Indikativ Präsensperfekt ("Perfekt") and Konjunktiv Präteritumperfekt ("Konjunktiv II der Vergangenheit"; for the IDS terminology, see Konjunktiv und Tempus):

Er hat keinen Alkohol trinken dürfen.
He was not allowed to drink alcohol.

Hätte ich doch nur Alkohol trinken dürfen!
If only I had been allowed to drink alcohol!

However, the difficulty in this case does not arise from the issue of the scope of the negation (nicht dürfen versus must not), but from the fact that dürfen plus negation is also used to express a suggestion or a request (see Duden 1b and DWDS 1c).

Du darfst ihn nicht so ernst nehmen.
Don't take him too seriously.

 

So darfst du nicht denken!
Don't think like that!

The original sentence uses Konjunktiv Präteritumperfekt, i.e. the perfect with the auxiliary in the Konjunktiv II. It is not about permission, but about a suggestion that has been ignored: It would have been a good idea for the boy not to speak to them, but he did in fact speak to them.

Der Junge hätte nicht mit uns reden dürfen, das ist gegen das Gesetz.
The boy should not have spoken to us, it's against the law.

Of course, the sentence in itself is perfectly ambiguous and could also be used to express that a permission would not have existed, if not for a certain condition.

Der Junge hätte nicht mit uns reden dürfen, wenn er nicht der Sohn des Häuptlings wäre.
The boy would not have been allowed to talk to us if he wasn't the chieftain's son.

I think this is just a case of ambiguity, or polysemy: dürfen usually expresses permission and nicht dürfen a probihition. The latter is possible because the negation has scope over the modal, as expressed by the paraphrase in quotation marks.

Er darf keinen Alkohol trinken.
"Es ist nicht der Fall, dass er Alkohol trinken darf."

Compare this to the English translation, where the modal has scope over the negation.

He must not (is not allowed to) drink alcohol.
"It is necessary that he not drink alcohol."

Note that because English modal verbs are defective, only be allowed to can be used in the past. Two examples with Indikativ Präsensperfekt ("Perfekt") and Konjunktiv Präteritumperfekt ("Konjunktiv II der Vergangenheit"; for the IDS terminology, see Konjunktiv und Tempus):

Er hat keinen Alkohol trinken dürfen.
He was not allowed to drink alcohol.

Hätte ich doch nur Alkohol trinken dürfen!
If only I had been allowed to drink alcohol!

However, the difficulty in this case does not arise from the issue of the scope of the negation (nicht dürfen versus must not), nor from the subjunctive. Rather, dürfen plus negation is used here to express a suggestion or a request (see Duden 1b and DWDS 1c).

Du darfst ihn nicht so ernst nehmen.
Don't take him too seriously.

So darfst du nicht denken!
Don't think like that!

The original sentence uses Konjunktiv Präteritumperfekt, i.e. the perfect with the auxiliary in the Konjunktiv II. It is not about permission, but about a suggestion that has been ignored: It would have been a good idea for the boy not to speak to them, but he did in fact speak to them.

Der Junge hätte nicht mit uns reden dürfen, das ist gegen das Gesetz.
The boy should not have spoken to us, it's against the law.

Of course, the sentence in itself is perfectly ambiguous and could also be used to express that a permission would not have existed, if not for a certain condition.

Der Junge hätte nicht mit uns reden dürfen, wenn er nicht der Sohn des Häuptlings wäre.
The boy would not have been allowed to talk to us if he wasn't the chieftain's son.