Wrong. They are slaves under the 13th amendment of the US, which explicitly allows slavery as punishment for a crime. Some more on this:

The US currently operates a system of slave labor camps, including at least 54 prison farms involved in agricultural slave labor. Outside of agricultural slavery, Federal Prison Industries operates a multi-billion dollar industry with ~ 52 prison factories , where prisoners produce furniture, clothing, circuit boards, products for the military, computer aided design services, call center support for private companies. 1, 2, 3

• The US has the highest incarceration rates in the world. Even individual US states outrank all other countries.