"No," the guard said, "you're wrong. Slavery was outlawed with the exception of prisons. Slavery is legal in prisons."
I looked it up and sure enough, she was right. The Thirteenth Amendment to the Constitution says:
Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
Well, that explained a lot of things.
Assata Shakur, Assata, p.64