It's entirely possible for a customer to use far, far more bandwidth than the service is set up to deliver to him. Such customers ruin the service for all of the other users and/or wreck the business model for the provider.
The simple and obvious solution is to cap usage. Set the cap high enough so that the typical user won't ever notice the cap, so that only the statistically extreme users are affected. 250 gigs per month seems in the right ball park for that, mroe or less.
Perhaps someday they'll offer various tiers of service with different caps for different prices. That way everyone could buy as much service as they actually use, and not have to pay more to subsidize the extreme users who use far more than they're paying for. That seems more reasonable to me than the collective pricing scheme where everyone pays the same rate regardless of their usage.
Erm. Phone/cable companies often fall under public utility laws. Even in areas that offer multiple DSL providers, often there is just one telco that simply leases capacity to the other DSL companies. They do not do so out of the goodness of their hearts, it's mandated. By being given a legal monopoly of a service, they must agree to certain terms. One is manditory infrastructure maintenance and expansion, in exchange for tax breaks, grants, subsidies, and govt contracts. When you are a legal monopoly, it is more profitable to make resources scarce and charge accordingly, than to invest in necessary infrastructure. Hence why the mandated incentives. Comcast is quite known for pocketting the cash and not maintaining their infrastructure as they are required. Financially, you can't beat getting paid to do nothing. FCC and very public utility boards do have the ability to smack Comcast, but usually choose not to do so.
Comcast has also started to gauge in an interesting practice. Say person A downloading a file from person B. Comcast has an algorithm, to defeat P2P networks, to forge TCP reset packets. In other words, Comcast intercepts the TCP stream, forges two packets. One they send to person A, saying person B cancelled the connection. The other they send to person B, saying person A cancelled the connection. Dropping the connection would cause both computers to automatically attempt to reconnect and continue doing so. Forging the TCP reset packet makes both computers stop trying, saving bandwidth. It may also be illegal.