"You signed a contact blah blah blah whine whine whine"
A stipulation in a contract doesn't mean its fair or enforceable.
Want some examples? Look no further than Apple's EULA. It states you can't modify iOS to do what you want with it. EFF challenged it and, guess what? Now modifying (jailbreaking) is perfectly legal.
Look at some things that have happened with cellphone companies.
ETFs were challenged in court. The carriers lost, but settled in a way so they keep doing it.
AT&T's mandatory binding arbitration clause has been thrown out of court multiple times (still making its way through appeals and such), yet AT&T continues to put it into their contract.
Just because something is stipulated in contract, again, doesn't mean it is enforceable.
When you buy a car you sign a contract. Imagine if the contract said you could only use Costco gas and you had to park in a garage every time the engine was off, and if you didn't. your monthly payment essentially doubles. Do you honestly believe that would stand up as enforceable in court? Absolutely not.
Just about every major ISP in the US now has some sort of dedicated traditional video service, like cable TV or U-Verse or FiOS TV. Imagine if those services put a stipulation in the contract that stated your monthly fee would double if you used 3rd party video services like iTunes, Netflix, Xbox Live's Zune video services, or even streamed CNN from the website instead of watching the channel. Do you think that would be ruled as enforceable in court? Of course not.
AT&T is attempting to do exactly what I stated above. Theres a reason AT&T was broken up a few decades ago. These major wireless companies as well as the wireline companies need to be broken up again.
While you are correct in principal, the reality is that AT&T is going to detect people who are tethering without paying and charge them for it regardless of wether or not any of us feel it's fair or enforceable.