So other than the progeny part, how is prison labor not exactly what you said? People are literally captured, and based on overturned convictions at least some percentage are innocent. These people are then forced to work and can have their sentence extended arbitrarily by bureaucrats and not the court system. Based on the California example the system even tries to keep people working when given a court order to release people. The only thing preventing the in perpetuity part is basically the good graces of the bureacracy.
I do agree about the overuse of the term in other situations however