The percentage of those who root vs those who don't could very well be worth the carriers time and MONEY to drop the rooted users. Don't forget the throttling announcement a month or two back, that is clearly targeted towards those who tether (via root).
I fully agree we should get everything we are due, and by due I mean the money we pay monthly to the mighty carriers.
It is not illegal to root your phone, but the carriers TOS can and do say you are not allowed to tether without paying for it. I don't agree with their position, but they do have the ability to know more than we think. They can monitor, shape and inspect traffic. Cisco has the routers capable and the carriers are buying them.
Its a game of cat and mouse and honestly if the carriers didn't bloat the frickin phones up I would be happy.