Stepping aside the fact that I think most everyone here is playing fast-and-loose with the “slave” terminology here… Why do you feel prisoners doing low wage labor to be wrong?

Practically everyone in human history since the dawn of time has had to go out and produce something of value. Why, all of a sudden, should a murderer or rapist get to sit on their ass and consume what we all produce? I find nothing questionable about a humble job for them at all.