Well, I don't really know about the US, but the prisoners up here in Canada are paid for their work. Not very well and the system of having them buy daily necessities isn't great now, but apparently they've gone on strike to protest that last year. If the US is forcing their prisoners to work for no pay then that might be something they need to change.
Although, I was thinking outright slavery since it reminded me of news reports in June talking about slaves being used in Thailand for the shrimp trade. In that case, the slavers there were also murdering their slaves, in addition to forcing them to work.