You can use either, in my opinion. It's a meaning-focused question.
It could just as well have been:
Wenn man öffentliche Vekehrsmittel wie Bus, Bahn, ...
Menschen, die ein öffentliches Vekehrsmittel ohne Ticket nutzen...
"Beamte" is declined like an adjective.
It has different forms depending on whether an article preceeds it.
It is die Beamten, but without article just Beamte.
The reason it is not declined like a Substantiv is the origin of the word. It is originating from an adjectival participle form of an old verb. Since these are declined like adjectives, this word kept its declension. This is typical, although not obvious.
Other examples are "Fremder", "Angestellter", "Reisender".