I'm not necessarily defending the mandate, but maybe I can clarify the concept behind it.
It's not a case of legislators saying "it would be better to have less phone theft so let's try to reduce it this way" - instead it's more like, users want this, but don't have the bargaining power to compel the phone makers to build it in or the telcos to support it.
Without the mandate, the makers and telcos profit from theft: the stolen phone user (not necessarily the thief) pays phone charges, the victim has to buy a new phone, and thieves have a continuing incentive to steal them. With the mandate, the phones are less valuable to thieves (and to robbers - a personal-safety gain), and the telcos can't profit from the forced transfers.
Again, not saying it's a good or bad policy (can someone remote-kill my phone when I still have it?), but these are the considerations - a kind of market-failure correction.