That case was about the (private) prison system itself profiting from the number of inmates increasing, not from labor the inmates were forced to provide.
You don't think the fact that the prison's economically depend on slave labor to function has any side effects on how the justice system works for those slaves?
It's certainly possible.
But the courts doing the sentencing and the prison owners are typically completely separate entities.
It's not just one "justice system" run by one group of people.