Detecting bittorrent isn't that hard, and neither is detecting most popular p2p traffic. It's CPU intensive though, requiring something like snort to run basic protocol heuristics on every packet. Just blocking as much outgoing udp as possible would be the poor man's solution to a network policy like that. There's also ways to cloak network traffic, of coarse.

Of course, in practice, all of the bandwidth can still be sucked up by p2p through VPNs which are probably allowed through the network, so I'm not really sure what the problem those restrictions try to solve is.