I like how we instinctively characterize prisoners as slaves. Seems like it trivializes actual slavery, but I have been out of school for awhile, so the conventional wisdom is probably different now.
Traditionally, prison labor is considered distinct from slavery for the same reason that prison is considered distinct from kidnapping and forced confinement: that it reflects just punishment for which someone is duly convicted. Whether that actually applies to most people in prison today is certainly up for debate, but to tell you'd mostly need to look at the judicial system (or society at large), not at the work camp.