We don't lock murderers up to punish them. We lock them up for the good of society, and we
deny them their freedom until they are able to demonstrate that they can use it responsibly.
(At least, that's the theory. "Punishment" is nothing more than vengeance with permission, and
doesn't help anyone.) So there's no objection to extracting as much good for society from the
situation as possible. Whether it happens to coincide with the prisoner's desires is
immaterial, and denying someone something they like doing, when it would be useful to
everyone, simply because "you're supposed to be being punished" is stupid and irrational.
Indeed, far better to put prisoners to work doing whatever they most want to do - that way
society benefits most from its incarcerated members.
(In any case, a fair few prisoners *want* to sit around all day doing nothing, getting into
testosterone-triggered territorial fights and spending the rest of the time smacked up into
oblivion - which means that the current system is catering for their desires almost perfectly.
Why are they more deserving than Hans Reiser?)