Happens in Canada as well under the Corcan banner and Trilcor in Ontario.
They say it is voluntary, but often if an inmate refuses he does face unofficial repercussions ie: refusal is used as a way of affect parole decisions, security adjustments, etc. Also the better work positions go to those who are lifers or have longer sentences. The purpose of the program is to provide job skills to those getting out, but jobs that actually provide job skills go to those who aren't getting out anytime soon, if ever.
Pay is also crap, with Trilcor in Ontario inmates are not paid. They are provided additional snacks at the end of the week. With the federal program Corcan they are paid under $6/day. Inmate's especially on the Ontario side should be paid more. Ontario inmates are those with a sentence under two years. Often when they are released they have no supports or housing which means heading to the shelter system and more than likely more crime.