What amounts to poaching other people's resources works well right up until you drive that other party into the wall and force them to spend a crapload of money for which they receive nothing in return. That is, they don't receive any renumeration for the additional expense - but you do!
This is the base problem with all overcommitted services where the business model is predicated on fractional use of maximum possible resource consumption. When that model is violated costs go up dramatically. This is ok provided the person who has the cost also gets the revenue that is occasioned by the violation of the original model.
The entire argument hinges on the idea that Comcast's model of over-subscribing is falling down in the face of increased traffic.
But who in their right mind didn't realize that internet traffic was going to continue to grow? Frankly, I'm sure Comcast saw this coming. They had ample opportunity (and money) to make appropriate upgrades to their infrastructure; instead, they figured that they could simply keep the money, and then blame the users and other content providers for using "too much" bandwidth, passing the costs along to them.
Long story short, they COULD have kept the old model going (as other ISPs have, so far), but why would they? This strategy is more profitable, and no one is telling them they can't do it.