First, I did not argument that I was NOT violating their TOS, if their TOS says I cannot tether without a specific plan and I do so, then I clearly violate their TOS. What I said was that IMO, the provision in their TOS may not be enforceable. If it is not enforceable, then I have every right to do so. I further said that a court would have to be the one to make a final determination as to the enforceability of this provision.
And you have case law to support your arguments? How do you know what a court would do? I certainly don't. I would contend in your first analogy, that if you owned both houses and lived in both houses you would be able to run a cable (notwithstanding the logistics involved). If you rented one, then you would not have such entitlement. Nothing is "plain and simple" when it comes to court decisions. And you're right that people argue the law when it is convenient for them. That's why there so many lawyers and law suits in this country. What may seem "plain and simple" to you is not necessarily "plain and simple". I will say that in your scenario, I would actually take your position in the argument. But I can certainly see that the other side can make some very valid arguments and actually stand a fairly good chance to prevail.
Your analogy of the BMW is a poor analogy since 1) the automobile is still owned by the leasing company (though you can certainly argue that AT&T still owns the towers and data flow), and 2) a person that leases an automobile still has certain rights and privileges that had been the subject of much litigation and legislation.
The relationship between jailbreaking and TOS is simply that just because a company says it's illegal and puts it in their contract does not make it so. As I mentioned, Apple contended that jailbreaking was illegal. Well, just because they said it was, didn't make it so, did it? Jailbreaking is legal. The relationship between the two is that just because AT&T says tethering without a specific tethering plan is a breach of their contract, doesn't necessarily make it so. Yes, jailbreaking and tethering are two separate issues, but the contention by a company (regardless of what company it is) that something is illegal by virtue of the fact that they put it in their terms of service or contract is the same. If there are differing opinions, then it is up to a court to decide which party is right.
AT&T has not been sued because they haven't pushed the issue. If I violate their TOS and they do nothing, why would I sue? Law suits, particularly in Federal Court, is extremely costly. If I was able to bring a class action suit against AT&T, could I recover costs? What would the damages be? AT&T has to evaluate what they think their chances of prevailing in a court would be if they pushed the issue. Then weigh this against the revenue they would gain of lose by winning or losing the case. Since they can work out the deal with Apple to incorporate their plan into the firmware and since the percentage of people who violate this TOS is relatively small, it may not be worth the risk and cost of litigation.