I can assure you that (in the US at least) when you buy a car, pay off any financing, and ultimately hold the title free and clear, you do in fact own the car.
Titles to ratty, broken down, cars are bought and sold every day... aren't they? And, just because it's shiny doesn't mean it's a great car either. It could be a great sculpture.
I think the issues at play here are more about service and warranty contracts as well as buyer expectations. Having the title just means I own a giant hunk of metal and plastic, no?
Radios these days are software defined. They can sit on the same chip that uses a trusted bootloader and also provides an engine control module. Marketing departments can and will bend and pervert engineering specifications to highlight the consumer benefits and minimize the profit motive.
QED
Assuming you can even get to the antenna without special tools... you're just going to cut the antenna so short that the radio can't get a signal? On a hunch?
What if the SOC in your vehicle has to download a rolling encryption key every week in order to activate?
Going back and forth with measures and counter-measures besmirches the point. Cars have to be serviced and this may be considered just another aspect of it.
I'm not suggesting that this sort of thing is necessarily a good thing(tm) but, it may be viewed as such by some and it's possible.
Isn't it the consumer's prerogative and responsibility to decide?
Not to mention, lawsuits and PR nightmares can be factored into a business case analysis. VW actually went ahead and defrauded the US government in diesel-gate and they're still an almost $100 billion company.
The prima facie reason is that in case of an accident the car should be able to request help even if everyone inside the car is unconscious. The other reason is, of course, that document you signed at the dealership stated that you've only purchased the _right_to_use_ the car... not the actual car.
Have fun with your rental :-) You should have read the fine print.