As far as doing hobbies (at night, Bein Hasdorim, etc.) Bochurim are definitely free to do these. Obviously they won’t be considered Masmidim by drumming three hours every night, and they shouldn’t be. Perhaps there is this ???? ?????? that’s keeping people from doing hobbies.
I happened to have managed more than one hobby, and come to think of it, even my own Limudim during certain free time.
Mashgichim of some Yeshivos would try to discourage other activities in their quest to turn you into a Masmid. Usually, once they realize that a particular Bochur won’t become a full time Masmid they stop discouraging him from doing his hobbies.