I agree with Protophore. Comcast has to know darn well that broadband traffic trends upward, and within a couple years this "data tax" is going to be sucking money from everybody - Bill Gates to Aunt Edna.
If the rest of the telecoms follow suit, it will soon put an end to the concept of the Internet as a democratizer and turn it into another class-based system.
Because customers' monthly fees go into R&D, there's an implied contract that their services will continue to improve. This is a violation of that implied (not written or legally binding) contract.
It'll be a scathing indictment of the state of U.S. capitalism if a competitor doesn't step in and offer customers better service for lower costs.