This oversimplification is a fiction. Slavery is still legal in the United States, so long as it is pursuant to a criminal conviction and if it is limited to compulsory uncompensated labor—and indeed that is precisely the system America maintains today.
The 13th Amendment, as enacted, reads “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.”
Slavery is neither a cruel nor unusual punishment according to the Supreme Law of the Land, nor historically has it been considered that. In the 1700s and early 1800s, Americans viewed compulsory labor as a way to fight vagrancy and to rehabilitate such idleness.
However, the states began to understand the potential for revenue generation from prisons in the 1800s—compulsory labor and the sale of prison products became a means to offset state costs. To be sure, the Virginia Supreme Court in Ruffin v. Commonwealth (1871) declared that prisoners were the “slaves of the State” within a compulsory labor system.