Most universities block certain traffic (IRC, P2P, IM, etc) because it takes up a lot of bandwidth when you have thousands of people on one network doing the same thing. Many universities and colleges in the US have only a T1 connection and even thought that's a fast connection, it's still not that fast when you split the speed up over thousands of students. And universities with T3 connections generally have even more students, so it doesn't really offer that much more speed per student. By blocking common download apps (by blocking their ports), it reduces the bandwidth being used for non-school related things. It's better to have full speed for people doing school work than for people downloading music for example. Beyond that, there's the virus issue. One person on a college network getting a decent virus can infect much of the network unless the university requires all students to use AV software. So there are two main reasons why they block the ports.
