[ninjagoo]

The US Constitution's 13th Amendment abolishing slavery specifically allows it for convicted people. [1]

You'll see from the definition of a "slave" [2] that prisoner labor specifically fits the definition of a slave, hence why the constitution makes an exception for it.

[1] https://constitutioncenter.org/the-constitution/amendments/a... [2] https://www.oed.com/dictionary/slave_n?tl=true