> Nobody should gain monetarily from someone being sentenced to prison.

I agree with your sentiment, but... are you aware of the fact that a large part of the US prison system is run by private corporations that have clear profit motives, and even seek contractually guaranteed occupancy levels?

Edit, link: http://annarbor.com/news/opinion/passing-house-bill-will-wor...

Money quote: "In 2012, [the Corrections Corporations of America] sent a letter to 'prison officials in 48 states offering to buy prisons from these states in exchange for a 20-year management contract and a guaranteed occupancy rate of 90%.'"