Simply banning books is a cost cutting measure, nothing to do with any political views on prisoners, it just makes it far cheaper to prevent illegal materials entering prisons, the poor state of libraries goes hand in hand with this cost cutting, and as this is an element of society who can't vote and are an unsypathetic target who the public won't care about it makes it easy for the politicians to get away with this. there is a good example linked in the comments section, and I know a couple of former prisoners myself, and you have far more faith in the system than me or them. Prison has never been about rehabilitation, and that doesn't look like changing anytime soon, it's a short sighted attitude that only prolongs the problems that the very people who complain about are willing to create themselves