In the US, paying prisoners is technically optional per the 13th amendment loophole.

Furthermore, the US has the highest incarceration rate in the world, and combined with for-profit prisons’ political contributions, there are perverse incentives and outright corruption to overprosecute poor and minorities, as evidenced by the skewed prison vs. entire US demographics.

And it’s ”their”

Yes, that is the point, they are not paying them fairly and pocketing the difference to subsidize prison costs. Meaning using prisons as an example of UBI being viable isn't a compelling argument.

Pointing out typos like that is boorish.