Forced prison labor doesn't have to equal slavery conditions of work. It can be to help inmates prepare to reintegrate society, or get them something to do of their days. The text said as punishment for crimes. At first I imagined forcing people to work hard 20 hours of day but it could be inmates having to choose work in the kitchen or cleaning or making objects that will be sold to the public a few hours a day. So I'm not sure if I want to totally erase this in the constitution.
Usually prisoners are paid (even if it's a very low wage) for jobs like that; this would eliminate unpaid labor. There are plenty of prison farms such as the Louisiana State Penitentiary, where inmates are forced to work hard field labor for no pay. Sound familiar?