There is a greater underlying issue here which seems to be missed.
I have paid £x to use O2's (or in this case AT&T's) network, not only that but I also had to partially pay for the handset.
O2 should not really give a damn about what device I use to access their network - sure, they may have sold me a handset with an Internet plan, but it is MY DECISION to use whatever device I see fit to use that network.
If I am allowed to use whatever device I want but it was capped to say 4GB, I would have no issue, but as it stands, I am not only paying to use the phone, but an additional bullshit cost to tether the phone which technically should be none of their concern.
That's a bit like saying "I bought a plane ticket, and if I want to cram multiple people, cargo, or whatever into the seat that's MY decision and the airline should not give a damn".
There's no underlying issue. You accepted their offer of a subsidized handset in exchange for entering into a contract to buy voice & data for that handset, or an equivalent, for a period of time. Your agreement pertains to that type of device only, clearly stated in the T&C's.
Now, if you want to argue that tethering charges to use your bandwidth is a dick move, or that mobile operators should NOT be able to discriminate, or charge more just because you own a certain device, then I completely agree. But neither of those are what you agreed to.
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."
I have paid £x to use O2's (or in this case AT&T's) network, not only that but I also had to partially pay for the handset.
O2 should not really give a damn about what device I use to access their network - sure, they may have sold me a handset with an Internet plan, but it is MY DECISION to use whatever device I see fit to use that network.
If I am allowed to use whatever device I want but it was capped to say 4GB, I would have no issue, but as it stands, I am not only paying to use the phone, but an additional bullshit cost to tether the phone which technically should be none of their concern.