The leasing isn't the problem. Under the constitution, convicts can be regarded as slaves - that's explicit in the anti-slavery legislation.
The state farm prisons are even older than that.
There were chain gangs.. but who cares about the history of prisons and camps in their own country?
The important pov is that of the prisoner - right?
So wtf is the difference to the prisoner for whom s/he has to work? Just being de jure or de facto slaves is what is important.

Let's not be fooled by the idea of voluntarily working in a closed system either - you must work - and you do want to do something - a 25 year sentence (3strikes) is a long time doing nothing.
But it is the choices available - and who's getting the rewards and the benefits?
And that is here the point - Are the shareholders of these companies not partners in crime since they are materially benefiting from the crimes that were committed? Yes. They are benefiting
enormously through punishing the prisoners - including the privatised prisons and any privatised services used by the prisons.