[chris_j]

You're absolutely right about the T&Cs. Presumably AT&T are aware that some people are prepared to pay more for tethering and therefore write the T&Cs such that can price-discriminate in order to get more money out of those people for potentially the same service. The sad thing is that people probably signed up in the expectation that the T&Cs would not be enforced and are getting a rude shock.

To slightly modify your airline analogy, this is like going on a business trip and flying economy. The airline scans your baggage, notices that you have packed your suit and other work-related items and demands that you pay the business class fare. "Hold on a minute," you complain, "I am entitled to 20kg of hold baggage and 7kg of hand baggage, so long as it fits with certain dimensions and isn't dangerous!" The airline retort: "Read the small print. Business travellers with certain items in their baggage will be charge the business class fare, which will be charged to your credit card. Now, enjoy the flight and don't forget to pay attention to the safety announcement."