I found an interesting academic paper on the dilemma that Comcast faces with BitTorrent uploads: “The Interaction Between the DOCSIS 1.1/2.0 MAC Protocol and TCP Application Performance”:
“We have developed a model of the Data over Cable (DOCSIS) 1.1/2.0 MAC and physical layers using the ‘ns’ simulation package [2]. In previous work, we reported on the impact of several DOCSIS operating parameters on TCP/IP performance [3]. In this paper we extend those results by looking in greater detail at the impact that the MAC layer has on TCP performance when using the DOCSIS best effort service. We show that the interaction between DOCSIS and TCP exposes a denial of service vulnerability. By taking advantage of the inefficiency surrounding upstream transmissions, a hacker can severely impact network performance.”
In effect, several BT streams in the DOCSIS return path mimics a DoS attack to non-BT users. That’s not cool.