Not american so someone correct me if i'm wrong, but from videos I've watched prisons are only obliged to provide you with the minimum average male calories per day so around 2000, for some people this isn't going to cut it, especially if you want to work out, and considering how little there is to do why wouldn't you?

Many activities are now charged as well, some prisons brought in tablets instead of having books, these cost money every day you rent them. Phone calls cost money.

So it's not exactly forced but if you want to do anything other than sit in your bunk all day or watch the tv's you are going to need some form of income, be it prison work, family, or a prison hustle of some variety.