I don't think the medium matters. It's unauthorised access to a system - whether or not that system relies on Morse code, default passwords, or radio waves - that's the legal boundary.
That said, cars are notoriously vulnerable because people do stupid things like buy cars that let you start the engine by radio-signal. I am of the opinion that *access* to my car is nothing more than a window-smash away for someone who wants to do it. Thus a remote fob that unlocks the doors isn't the end of the world, and also requires a criminal act to happen. I don't keep anything in the car that's worth nicking.
However, I would posit that, from there, making a car start would be much more difficult and likely to attract attention (at minimum I would think you'd have to open the bonnet, somehow bypass / compromise the immobiliser, which is coded to my *physical* key and not any radio fob). I still wouldn't put it past people to have a way to do it, but you can't just load up RTL-SDR, with a £20 TV-tuner dongle, a small laptop, and record/re-broadcast the 433MHz frequency from my keyfob in my house such that the car would start and you can drive off (which is what you *can* do with the new cars).
For a start... my keyfob only broadcasts when I press the button, not when it's just sitting in my coat pocket in the coat on a hook inside my hallway.
And for those wondering - yep, you can pick up the signal on 433MHz. It's a simple plugin for RTL-SDR software to show it. It doesn't mean you can *tinker* with it as easily but from there it's just a matter of finding the actually-dodgy software that interprets the Ford key protocol from the net.
Guess how I know this is possible, and what dongle I bought last week, and what I've been using to look at everything from weather stations, to ATC voice and data traffic, to picking up Heart FM. I'm staying *strictly* on a receive-only device, however, but it's scarily easy to broadcast (e.g. a single GPIO on a RPi connected to an antenna and a different piece of SDR software - ugly, probably illegal because it's a squarewave broadcasting on all kinds of frequencies, but nothing a radio ham couldn't also do in seconds).
If you don't want something to happen, don't allow it to happen even in theory. This goes for everything from buying receive-only devices when you don't want to get into trouble because someone reads a comment on the Internet and thinks you're hacking the neighbour's cars, to installing apps with permission to use the camera, to buying cars that let you start them from nothing more than a radio signal.
My car can only open the doors on a radio signal. That's it. And that's nothing more than you could get in a few seconds with a skilled, or brazen, thief. If you don't allow the thing to be possible in the first place, then you don't need to worry about how to secure it.