If you want an example, the hellhole prison nicknamed "Angola" (because it's full of black people haha so funny) is a good case study for why this is slavery and not just prisoners paying their way through prison.
They're forced to do work or get solitary confinement, make $0.02/hr under inhumane conditions, punish people who get heat stroke, don't provide shade or fans or clean water (literally, they have to drink dirty water if they don't use their teeny wages to buy water) or anything, in temperatures that exceed 113°F in Louisiana. They're forced to water plants with paper cups, pick rotten fruit by hand, weeding by hand, it's all totally pointless shit an the cruelty is the point.