High-usage users are a drain on their profits, and they only provide service to us because we'd be a PR disaster if they tossed us and we ended up telling all our friends and family to switch to some other provider. On the other hand, we would obviously prefer much higher or non-existent caps. Comcast is understandably not amenable to that, as it'd require substantial investment in infrastructure improvements for minimal increased revenue. Whatever happens is going to be some kind of compromise, and the one they're proposing makes them additional money while also giving us "power users" an option besides "run into the cap and die".
I don't think that that's a terrible idea. Like you, I'd prefer higher caps for the same price, but the reality of it is just more complex. ISPs are strongly opposed to doing anything that would increase costs or decrease revenues, and customers are strongly opposed to doing anything that would increase prices or decrease quality of service. Something has to give, and in the short term that's going to mean pain for both sides. In the long term, technological progress and the limited (but present) competition between providers will mean that we'll probably end up paying about the same amount for a slowly-increasing amount of service. That means faster connections, higher caps and more options, just not as soon as you or I want them.
Disclaimer: I don't have Comcast cable, but I do have cable service from one of their smaller competitors.