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.
I am sick of reasoning like this. The purpose of government is to preserve your freedom to do something. If some users want something and cannot arrange it themselves, then they may just not be able to get it. It is not the government's place to mandate that everyone gets what some people want. That goes against personal freedom. It is certainly the government's place to punish thiefs---a person who deprives another of their freedom to control their property. The government should not mandate certain ways of arranging private (between a person and the phone company) affairs.
That's maybe what you think the purpose of government should be, but it doesn't really match reality. Consider the whole section of labor laws, for instance.
But we can counter that it's specious to claim customers are clamoring for this but not getting anywhere with manufacturers. Apple's Find my Phone feature is already one step toward addressing this issue, and there are a handful of third-party apps on the market that do similar things. These market solutions will continue to get better over time if they're popular.
And of course, there's the question of why it's the manufacturers' responsibility to address theft in the first place. Jewelers aren't required to engrave and register all their necklaces.
Cars and houses can have alarms, and customers decide whether they need them or not. We do not require that all cars and houses come equipped with them. Wallets can be attached to a chain or placed in the front pocket. We don't require that you can only purchase a wallet with a chain.
Unlike childrens' toys that require battery covers to be screwed shut, or cars that must have seatbelts, the theft of a device does not seem to be a public safety issue. Your decision to own an expensive phone and take it out of your pocket at the train station seems no more necessary of regulation than your decision to wear an expensive necklace.