I think the point is that Comcast is already engaging in tortious interference, and then trying to charge the upstream provider for the privilege. Sure, they could file suit on those grounds, but the countersuit would follow very quickly, and it would be joined by everyone.
The concept of "Tortious interference" [1] comes to mind but I'm certain there are other legal theories that could be employed by Comcast.
[1] http://en.wikipedia.org/wiki/Tortious_interference