[jollybean]

" because you want to have your underclass ready to hand for slave labor. Or, generally, to repress them."

This is not unsubstantiated.

Moreover, it's upside down:

The economic labour output from US prisons is negligible and has no material effect on the GPD or industrial basis.

... and those prisoners, were they outside of the prison systems, in 'regular jobs' - would add tremendously more to the GDP in terms of productivity.