Re: Yeah, its about protecting your stolen phone - seriously?
No, that can't be it. If they believe that a phone is going to be used as a trigger for a bomb, all they have to do is ask (or require) the phone company to disconnect that phone number (or better, for evidence-gathering, to divert the phone number to their evidence-gatering facility). If they can't do this today, it's a relatively uncontroversial bit of legislation to fix it so they can.
On the other hand I can see why they might want to permanently kill every phone within 100m of a demonstration at which police brutality was happening. Other things like that. And of course, it's completely impossible that some hostile foreign government might get its hand on the kill switch for every mobile phone in the USA. Isn't it?