Why? This is essentially a signed kill message and message signing has generally been very secure, good luck getting anyone's root key.
Yeah, just like Sony Playstation's, or the Blu-ray's, or Debian's root key have never been compromised neither...
In a perfect world, were everything is perfectly implemented, such a "remote-kill-switch" could probably work only as intended, and the only reason to be afraid would be potential abuses (Government authorising warant-less remote shutdown because of newer laws against cyber pedo-terrorist pirates, Cops abusing the system for their own gain, etc.)
In practice, you know that the implementation is going to be imperfect and flawed. Probably 6 years after its release someone at a hackers' conference will demo an exploit that involves sending a malformed data packed on the same frequency as the tire-pressure detector talks to the car, because the car's subsystems weren't correctly isolated.
Beaten 6 months later by another team which discovers that the "oh-so-easy-to-hack" on-board entertainment system [complete with wifi/bluetooth/4G online access], actually *DOES* talk on the same vehicle-wide network as the car's subsystem even if nobody in his right mind would ever design such a system.
And it doesn't matter, because your local car-jackers had the root key anyway from day zero, because they bought it from some foreign thief, who bought it from the russian mafia, who got it "leaked" from the FSB, who got it because one of the engineer designing the whole system was actually one mole agent planted by them. (And then Snowden will reveal that the NSA unsuccessfully attempted the same. But as their mole got caught, the NSA resorted instead to getting one of the real legit engineer drunk).
Cue-in tabloid story of a cop who blocks the car of a love intesress' current boy friend and courting competitor....
Probably a bit of tin foil around the antenna would do the trick, maybe it won't work on getaway cars but police stop runners, DUIs, people driving the wrong direction and a lot of other loose cannons probably wouldn't have done that.
You probably will get a whole range of solution, between simple tinfoil to jam the antenna, to simply using older cars dating before this system, to complex hacks that look completely legit on the radio wave (like correctly answer to pings and will acknowledge a remote kill order), but do not actually enact the kill.
Probably privacy and security savvy everyday users will try the former, and probably get busted and heavily fined for it.
While criminal will try the later solution (car predating the system, or hack that quacks like the duck, walk like the duck, but aren't actually the duck) with great success.
Oh and all military aircraft have kill codes today I think, want to do a runner with a US jet to Russia? Methinks you'd never arrive, even if you could avoid being shot down. Missiles definitively have self-destruct codes, now if it was this totally insecure why would we build systems to totally cripple ourselves in case of war?
There is a small difference between the military (who have plenty of budget and won't mind spending it on the top of the line. Their things might be completely overpriced, but they can afford proper audit) and mass produced goods like car which have to be as dead cheap as possible (be ready for the kill-switch's firmware to be outsourced to the cheapest asian contractor).
And I'm ready to bet that, deep at the MSS and at the FSB, someone DOES know the root key to US missile remote deactivation.