Operators in Brazil have done this forever. The phone's ID is probably already sent along when you activate the SIM card, why would a company not do this?

They do it here in Sweden as well (or at least they did ten years ago). There's really no reason not to do this.

NOt that it is a /good/ reason, but the cost involved I implementing the system is often the reason listed as

Verizon will block stolen phones by ID as well. If you go to buy a used phone you have to call them and ask if it's stolen or not.