They are literally forced to do unpaid labor. There are also prison jobs that do pay, but extremely low amounts and those are generally optional. Prisons do also offer volunteer work, it being voluntary makes it not slavery.

Incarceration on the other hand is a different topic altogether and I think the US system is incredibly harmful and creates an environment that produces recidivism. The Nordic system of rehabilitation is far better.

There is actually a pretty good subplot about this in Gone With The Wind. It’s slavery.