Interestingly, in the US, slavery is explicitly permitted by the Constitution as a punishment for crimes. (Though whether prison labor should count as "slavery" or as a different form of "involuntary servitude" is an interesting semantic question.)

"Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction."

To be fair the US already do a slavery, prison labour is a thing.

To be clear, that's exactly what I mean. Every perverse incentive that nudges for-profit prisons towards manufacturing prisoners is magnified by turning individual prisoners into revenue streams.

It's a US constitution problem. Slavery of prisoners is still legal.

There's a film on it https://en.m.wikipedia.org/wiki/13th_(film)

This is less of a gotcha in the context of locking people in boxes, which we also normally don't do.

Locking people in a box is incarceration. Locking people in a box and compelling them to perform labor from which the incarcerator profits is slavery.

The fact that imprisoning people costs a lot of money is one of the few things keeping the U.S. prison population in check. That's not a problem that needs solving.