I don't believe the prison work programs are forced at all, I think they have a choice to do it. Instead it's a chance for them to get work experience/vocation education that they might not have received.
Work credits could be seen as getting a shorter sentence by showing that you can behave like members of society (i.e. displaying evidence of your rehabilitation). They normally give credits like this for general good behavior, too (again, evidence for behaving in society, at large).