What's the idea?

Can't quite get my head round this ban on books being sent to prisoners.  Is this a security matter - it is said that parcels are an easy way of smuggling contraband into prisons - but surely this is just a matter of having an efficient examination system for incoming mail?  Then again, it has been suggested that it is a question of privileges - you have to earn the right to be allowed to have books.  If that is indeed the reason what happened to the idea that prison is supposed to be rehabilitative? Of course, I suppose there's always the prison library.