I also have heard carriers generally do not allow it (i.e. software makers pushing updates without the carriers permission). I have heard the reason is basically support costs. If they have not had time to test and vet your update against all the models running on their network, and you brick a phone (or a million), those customers are calling the carriers and they are probably pissed. They wouldn't know how to contact Google even if the thought of doing so wasn't so laughable.