In many states it's, at the very least, coerced.
Having a prison job often comes with deals of better behavior and a shorter sentence (!!!). When you're being told that just working for 2 dollars an hour might lower your sentence from 20 years to 15, do you really have a choice?
For example, in Georgia, prisoners often work outside of the prison for well below minimum wage in order to earn "good time". This means they might get more visits to their family. It also increases their chances of parole. However, the labor is coerced as well. Showing up late or not coming in results in in-prison punishments. So, many prisoners work in cotton fields or McDonald's on the promise of an easier life, while most of their wages are siphoned away and businesses get to pay very little.
This is a mockery of the term "slavery". It is no more slavery to be coerced to work for a shorter sentence than it is slavery to be coerced to follow the law to avoid prison.
"Behave a certain way and you will be imprisoned for less/no amount of time" is not slavery unless the law is slavery. The full term imprisonment is just, and being able to shorten it is a privilege.
> The full term imprisonment is just
This is a contested assumption. Prisons and penal systems in US as I understand it are for profit.