In fact the 13th Amendment explicitly calls out being convicted of a crime as an exception to the "no slavery" thing.
The full text of the amendment:
> Section 1. 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.
> Section 2. Congress shall have power to enforce this article by appropriate legislation.
By word count, 30% of the amendment allows slavery under specific circumstances, but somehow that seems to get left out of the curriculum of most american civics classes (that I've seen / gone through anyway).
The full text of the amendment:
> Section 1. 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.
> Section 2. Congress shall have power to enforce this article by appropriate legislation.
By word count, 30% of the amendment allows slavery under specific circumstances, but somehow that seems to get left out of the curriculum of most american civics classes (that I've seen / gone through anyway).