Would you feel better about it if the prisoners' situations were identical, but the profits went to the government or to reparations for victims of crime?
The only correct answer is "if you employ people, you pay them." This includes people currently serving jail sentences. Going to jail is meant to be a punishment. You're losing months/years of your normal life. If people wish to offer convicts a way to help mitigate this by offering them employment, then they shouldn't get the ability to employ them at a discounted rate, because that's punitive to people who are not in jail.
The courts already have a way of doing this that doesn't involve slavery.
I think the biggest issue everyone seems to have is the idea of below market wages being paid to the workers.