Comment by larkost
In many cases the work is not really voluntary, there are sanctions for not taking it. Prisoners in some states are regularly put into solitary confinement for not "volunteering" to work these jobs (a punishment that some areas deem torture). With that amount of coercion I can't see them as voluntary, and so the slavery label is awfully close to the mark.
In those situations, I would agree that is pretty damn close to the slavery mark.
I've worked with a lot of prison facilities though in many states across the US and a few international, and have never seen that. That's not to say it doesn't happen of course, but out of curiosity do you (or anyone else) know of any facilities/jurisdictions that do that?