Comcast, and other ISPs advertise with certain upload and download rates, conforming to simple DOCSIS capacity numbers. However, they miscalculated and found that there is more to the Internet than browsing, gopher, and email. Heavy-users broke their excel return-on-investment predictions and marketing campaign promises. Their flat-rate offers simply became too popular for the capacity that was bought and installed.
Comcast’s BitTorrent Throttling Acceptable? Not Quite! TorrentFreak
This really echoes the point I've been making for a couple of years now. Charter also uses bandwidth throttling to cheat us of the bandwidth we pay for. I think a class-action lawsuit is an appropriate response to this practice. There is no other way to describe it, other than simple fraud.