The 13th amendment was suppose to abolish slavery from ever existing again. But with riots and protests occurring, prisoners do not think slavery ended. Without getting pay, the work when they serve their time. They think they don't get paid for working when in reality they do.
Inmates get forms of payments for serving their time for being criminals. For example they get a shelter. They don't pay a dime to live in their cells. They also get three meals a day and free healthcare while serving their time, and free entertainment. They also get free weights and they get to learn skills for when they get out. Money isn't always in paper form, sometimes it's four walls and a roof.