Sorry for my ignorance...
But I'm wondering why a carrier would have to do anything when a customer takes a SIM card out of one phone and puts it in another. I switch SIM cards round all the time, even putting SIM cards into phones that are branded by a different carrier. As far as I know, and what has seemed to hold true in my experience, the carriers have no control (or if they do, they don't use it) over flicking SIM cards into different phones. So how is it different in the US of A?