Comcast's basic problem here is it wants it both ways. It wants to advertise all you can eat connections of the highest speed — because that sells so much better than alternatives like metered pricing or explicit bandwidth caps. But it doesn't want to deal with the consequences of the user behavior this sort of advertising generates. i.e., people using their Comcast connection all the time and expecting the advertised speed. Nor does Comcast want to deal with this the traditional way, by spending the money to build more capacity, then charging a higher price for the new “top speed.” Comcast, like any other profit-maximizing firm, would prefer to avoid expenditures and, if it must spend money to gain revenue, would prefer to minimize expenses and maximize revenue. Inserting reset packets to degrade the reliability of BitTorrent, and therefore discourage its use overall, is much cheaper than upgrading from existing hybrid-fiber-coax to fiber. So, as I predicted over a year and a half ago Comcast makes the logical choice and degrades traffic that eats bandwidth rather than pay to upgrade.
Note: It's illegal because the way in which they are blocking it is in fact a denial-of-service attack.