Netflash (ISP in Ontario, www.netflash.net) openly admitted they block P2P traffic in an email (their excuse is it is unhealthy). They provide a wireless service to rural areas. Portion of the email is below:

I'm assuming that this is through a peer-to-peer program, that allows you to download music, movies, etc for free.

This type of service, as well as many other unhealthy items, such as
viruses and spyware, are very unhealthy for any network, and a wireless
network is very sensitive to these unhealthy items. As a result we have
blocked many of these items to ensure reliability and maximum uptime.