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.