"Actors act at the theater."
Translation:Oyuncular tiyatroda oynarlar.
"Rol yapar" is just a missing alternative. Adding it now :)
So the issue here is clarity and context. oynamak does indeed mean to act (as it does many other things). Because you said the work "actors," the context of this sentence is very clear and most people will automatically assume you are talking about acting.
In the other sentence, the issue is of clarity. Very, very few people would ever assume that that sentence involved acting. They would immediate jump to playing (as in playing a game) or that they were dancing a traditional Turkish dance (as opposed to a Western dance, which uses a different verb). We are currently discussing adding it as an alternative, but I am a bit hesitant because it wouldn't be understood to mean "acting" in the vast majority of contexts.