Maybe because they want you to, well, learn in school and not be cruising the internets - not that censorship is good, but the kind of censorship you are talking about seems mainly aimed at eliminated distractions rather than eliminating "ANYTHING that could be considered offensive to ANYBODY." Of the list you posted, only a few sites are things that actually contribute to the school's mission, the rest are items that they might not want to spend bandwidth on. It's just like censorship at the workplace, you're there to do a job and not putter around the interent.