Short anwer
Here, gesetzlich is not declined, because it is used as an adverb, not as an adjective.

Adjective

the legal allowed cases → den gesetzlichen zugelassenen Fällen

Here, the cases are allowed and legal. Therefore, both zugelassenen (allowed) and gesetzlichen (legal) are adjectives, which must be appropriately declined.

Adverb

the legally allowed cases → den gesetzlich zugelassenen Fällen

Here, however, the the cases are legally allowed. Therefore, gesetzlich (legally) is an adverb that modifies the adjective zugelassenen (allowed). German adverbs are not declined.

Short anwer
Here, gesetzlich is not declined, because it is used as an adverb, not as an adjective.

Adjective

than the legal allowed cases → als den gesetzlichen zugelassenen Fällen

Here, the cases are allowed and legal. Therefore, both zugelassenen (allowed) and gesetzlichen (legal) are adjectives, which must be appropriately declined.

Adverb

than the legally allowed cases → als den gesetzlich zugelassenen Fällen

Here, however, the the cases are legally allowed. Therefore, gesetzlich (legally) is an adverb that modifies the adjective zugelassenen (allowed). German adverbs are not declined.

In this example, gesetzlich is not an adjective (it does not modify the noun Fällen), but it is an adverb (it modifies zugelassenen). Adverbs are not declined.

Short anwer
Here, gesetzlich is not declined, because it is used as an adverb, not as an adjective.

Adjective

the legal allowed cases → den gesetzlichen zugelassenen Fällen

Here, the cases are allowed and legal. Therefore, both zugelassenen (allowed) and gesetzlichen (legal) are adjectives, which must be appropriately declined.

Adverb

the legally allowed cases → den gesetzlich zugelassenen Fällen

Here, however, the the cases are legally allowed. Therefore, gesetzlich (legally) is an adverb that modifies the adjective zugelassenen (allowed). German adverbs are not declined.