Prison labor has good and bad points. On the good side, we would want prisoners to learn how to work so that maybe they could get jobs once they are released. We would also want them to do something valuable so we don't have to pay for them just to lie around all day or to do things that are fun. Instead, they could work and help make their incarceration cost less for us taxpayers.
But on the bad side, every job done by a prisoner is a job that cannot be done by a regular person. If we have prisoners doing truly useful jobs, that takes away a job that could be done by someone who hasn't committed a crime. It seems bad to do that.