That depends entirely on your definition of slavery. I don't believe that the average person considers a murderer making postbags for 25 cents a day is a "slave". Nor do I think it helps the argument for a focus on rehabilitation to suggest it is so.
Obviously as you change the crime and the task and the wages, peoples perceptions change. Which is fun, non?
Of course, substance possession and other victimless crimes are the useful place to begin.
And it’s crucial to differentiate between privatized prison labor and state prison labor. Privatizing human rights raises a major moral discrepancy as it stands. Profiteering of human rights with forced labor is a fair definition of slavery in my view.
Your argument concerns justice, though. Following emancipation, no time was lost in instituting the strong prison labor system the South known for. When there’s a profit motive, the entire system of justice becomes contentious. If seeking to get murderers off the streets (something I support) it’s critical we not conflate that civic duty with profiteering motives, lest we become the demon, or at least hypocrites. Of course, we popularly criticize the Soviet Gulag system on these same terms, among others.
There’s more, too. Getting a murderer off the streets is a categorically different goal from inflicting punishment or vengance. Getting a murderer off the streets is an easy decision. Punishing them is also widely supported, but still categorically different. If we intend a moral system, we must differentiate and actively acknowledge the difference.
There's yet a further subdivision to be made too, as there's getting a murder off the street (justice), punishing the murderer (vengeance/retribution) and stopping more murders (prevention).
After the Rwandan genocide and even in reaction to the residential school system in Canada, Truth and Reconciliation Commissions were set up to focus on the justice and prevention elements of the triangle and explicitly pass on retribution. [1]
The US system focuses, IMO, almost solely on vengeance to the detriment of prevention. The US has one of the highest recidivism rates in the entirely world at 75%, so if your goal is to stop people from doing it again, the US system is an abject failure. If you want to enslave and torture, it works great. Norway focuses on justice and prevention, and as a result, has one of the lowest recidivism rates in the world, at 20% [2].
