you're forgetting about "Prison Industries".
There are prison's that have "housing" jobs for inmates. They build roughly 40 houses a year (3 a month) and sell them for $35k-45k, while paying the inmates $.25/hour to build them. Then the inmates turn around and spend that money in prison commissary for items marked up 100-300% (over $4 for a jar of generic peanut butter that costs $1.60, $.58 for Ramen noodles, $3.50 for a $1.50 bag o generic bbq chips, $2.50 for a loaf of $1.20 bread, almost $5 for a bag of $2.80 generic instant coffee, ect, ect, ect...)... and not to mention the monopoly on the phone company's, where a 20min long distance call costs over $7 and a local call costs over $4 with a $3 connection fee. So if your call 'somehow' gets cut off after 1 minute, you gotta call back and your 20 min phone call costs $10 total (2 connection fees).
many times, especially in small towns where the prison is THE main 'industry', the prison officials (warden and secretary of corrections) are investors in the companies that supply the commissary as well as long-distance phone service providers.