It's closer to true indentured service, where a debt is being repaid through a fixed period of labour. I think it's wrong to compare it to slavery, although interesting how the racial make-up in prison supports the assertion!
I do think inmates are probably better off with work to do. I also think it's reasonable for society to garnish a chunk of the profit from their work to offset the cost of crime and punishment. Just my opinion, and at an abstract level rather than specific. I don't say the balance is currently right.
I'd be more sympathetic if the country doing this didn't have a history of imposing harsher sentences on minorities, criminalizing the ingestion of plant material, and a corrupt policing and judicial system.
The key difference is in the amount of liberties and the wages: prisoners in the US are forced to live in tiny cells for years and most importantly, are paid $0.25-1.00 per hour - a symbolic pay just for the prisons to be able to say they're paying prisoners and not using them as free labor.
Worse, the prisons actually get paid (a lot more) for providing these people to companies AND they get paid by the government to keep the prison system running.
