Jeez, I'm really not a Comcast apologist, I'm not sure how we got here ;-)

But in reality you, and I and Patrick above, as probably power users of our internet service, benefit from this overselling. If an ISP actually had to provide the entire bandwidth cap to all their users the only way they'd be able to do this is by raising their prices considerably. Instead they stick a high cap up there to discourage people soaking all the bandwidth with torrents and try to provide the best level of service they can.