Actually a hardware kill switch is really easy. All they have to do is over current a chip, or section of a chip and fry it out. Its easy to add that in if they want. Its not like you can repair 1 fused transistor among billions of others in a processor.
They shouldn't ever do this tho. If they can fuse hardware remotely. Hackers WILL find a way to do it, and there WILL be millions of bricked phones. Guaranteed to happen. Can you imagine the chaos frying 50 million phones on one day would cause? You can bet there will be thousands of people trying to do it, for that badge of honor(or shame, however you want to look at it); and they will most certainly succeed.
If you want a conspiracy theorist viewpoint. What happens when the government decides to turn off all phones for whatever reason. Sure, it probably wont ever happen. But that is FAR too much power to have.
Or the far more likely. What happens when they accidentally fry your phone instead of someone else's because they typed in the serial number wrong. Oops too bad, we killed all your data, so sorry, have a nice day.
-----------
Even the soft kill switch is an extremely bad idea. Its too much power to have the ability to turn off every phone at will. Sure sure, its not going to happen, untill it does.
Even with a soft kill switch, think of how long it woudl take to reauthorize 300 million phones, if someone decided to kill them all for fun.