They use programs like these 1. http://www.sandvine.com/products/p2p_element.asp,
2. http://www.ipp2p.org/docu_en.html, 3. http://www.p-cube.com/new_solutions/traffic_P2P.shtml They detect LW & can slow it's use. There's other techniques. I don't know which one Pipex uses ... OAH might have seen something about it.

Here's an example of how a p2p bandwidth shaper can work against Gnutella network: Bandwidth Shaper Sample - typical of how some ISP's use it (click on link) ie: giving priority to net surfing & low priorities to p2p sharing.