If you are jailbreaking for theft of services (i.e. being able to tether without paying additional carrier fees or installing apps that you didn't pay for) I can see where the carrier would get upset and require the phone vendor to make sure you cannot do this.
Theft of services? That's totally wrong! People pay for a service and it's insane to think that Apple or AT&T or whoever can tell them how they are supposed to use the data service they pay for. It's ten times more insane to believe that this is theft!
At best its interference with a corporate business model.
And as any market strategist worth his salt will tell you, business models that depend on restricting customer behaviour and usage are swiftly doomed to failure. If you want to make money you need to offer some value. If the customer finds ways of extracting extra value from your service, maybe you should consider charging more for it. Trying to stop the customer doing it is a quick ticket to nowhere. I worked in telecom strategy for many years and saw this a lot!
BTW - the whole notion of "I own the device, I should be able to do what I want with it" probably doesn't hold up legally.
On which planet? Even in China, if you own the device you can do whatever you want to it. You can hack it, take it apart, burn it, reverse engineer it... What you can't do, is do some of those things commercially like, reverse engineer the A4 SOC and sell clones of it. If you reverse engineer for the fun of it only it IS legal.
In IT, there are lots of instances where you buy hardware and pay for features to be enabled. Our sysadmin was just telling me the other day that we have a fiber-optic switch. We can plug things into the switch, but to turn the port on, we have to pay for the license to do so. This kind of thing happens quite a bit in Enterprise IT systems, but consumers are much less tolerant of these kinds of pricing schemes.
1) Corporate IT is governed by corporate contracts which are custom and completely different (and differently regulated) than retail sales contracts.
2)Even so the particulars of each contract can only restrict you so much. E.g. if you own this switch and there is an alternative way to activate the switch without violating the IP of the vendor that could be legal (depends on contract terms). Its just that you are very unlikely to find a way to do that.
In the case of a retail purchase and if no violation of apple's IP has occurred, there is nothing illegal inappropriate or immoral about you jailbreaking your device. Even if a breach of IP occurred, the guilty party is the one who reverse engineered the code, not the one who used it.
Corporations are trying to re-write the laws by re-educating consumers to accept a lot of unacceptable &#~! It's better to learn to protect our rights!