said by jester121 
:said by odog :Because your customers would defect from you. If I was a customer of an ISP, and my ISP only used AT&T for their internet connection my customers would get mad at me for not being able to use P2P. Also if you are a business level customer you buy raw, naked internet, with no restrictions.
Plenty of small businesses wouldn't mind AT&T filtering potentially illegal traffic from their network; it saves them some expense in installing filter/block software.
It all comes down to which direction the market takes. I'd say the vast majority of customers are going to shop price and speed, and if they find out afterwards that P2P doesn't work, they'll get over it.
So what liability does this give the Telcos since they want to play nanny? What if child porn slips through or even worse a unreleased copy of X-Men?
But obviously such a filter would be totally worthless and would exist only as an advertising-to-the-RIAA-and-MPAA gimmick.