See the above diagram which is excerpted from Allot.com. At least in China, the billing policy of telco companies often choose a coarse-grained top-price, such as 120RMB(about 14$) per month. Under such a price, the bandwidth users can do anything they like, e.g downloading sea-volume vedio files with BT, eMule, and etc. According to many statistics, traffic from such P2P-based file sharing eats-up about more than 70 percent of the total bandwidth of MANs, and might be even more at night.
So for a bandwidth access service provider, in order to increase its ARPU and better the network efficiency, it must find ways to categarize their traffic, such as non-P2P and P2P, and bill differently.
In other side, the traffic categarization helps defense the Denial of Service (DoS) attack to its network infrastructure.