Surely it shouldn't be about alleviating over-crowding in prisons (unless they sleep at night on said building site) or the prisoners' welfare.
They are inside to be punished for crimes committed, yes of course 'rehabilitation' is important but not at the expense of all the poor *innocent* buggers on the outside who can't get a job!!
Unemployment is high enough.

If someone seriously thinks there are employment opportunities - available jobs - on building sites, give the jobs AND the training to the people in the dole queue!

I think prisons here should take a leaf out of Joe Arpaio's book (without the wasteful lawsuits of course).