I'd Like to Understand ISPs-Ragoff-Moved from comments
ISPs block bittorrent (and other P2P) for two reasons. The first reason (and most important) is money.
The second reason is pressure from groups that protect copyright holders.
As indicated in my What is Bittorrent? article (linked at end of article) bittorrent may account for 40% of all internet traffic.
Also bittorrent makes use of peoples upload bandwidth, which was previously unused. Because it was previously unused many ISPs did not allot much to upload bandwidth and this has caused strain on their system. Bittorrent first began in 2001 and its use has soared in a short time and the ISPs have been unable to keep up. Accommodating bittorrent's use of upload bandwidth would require a large capital expenditure by the ISPs.
ISPs have also been under pressure from governments (pressured by the copyright groups) to help cut down on piracy of copyrighted items.
IMO both the copyright groups and ISPs have failed to keep up with technology. Both of them could benefit greatly from bittorrent if they used it properly.
I'd like to understand ISPs opposition to bittorrent. Comcast is in legal action with the FCC for throttling bittorrent traffic, and ATT and my own WISP have simply blocked it. If bittorrent is such a great protocol, why do the ISPs block it?