It might be worth doing if it’s of psychological or behavioural benefit to the prisoners ie it prevents boredom / fighting / depression, and inculcates an ‘employee state of mind’ (usefulness, discipline, and ‘earn to spend’).

That’s fine if the state was providing these activities as unpaid or low paid hobbies in addition to actual jobs.

The problem is that the state is exploiting the prisoners for profit. The profit should go to the prisoners doing the work.