I don't see why you would need to even prevent access to the interface. After all, there is quite easy access to any number of pieces of hardware on the car, that if modified incorrectly could easily cause injury or death to humans nearby. Are these car manufacturers as worried that I might drain all the oil from the engine, then drive around until the engine seizes? What if I mess with the brakes? How is changing the SW in the car really any different?
Quite frankly, it would be much better if the various pieces of firmware and IVI software were open source and available. Then third parties, like auto mechanics, could have access to this information and be able to do repairs. As it currently is, all SW repairs have to be done through the dealer. They are bad enough with proprietary diagnostic tools (why the "check engine" light is on).
IANAL, but I believe there have been cases in the past where car manufacturers tried similar tricks on hardware to prevent third party repairs and were smacked down by the courts. I don't see why SW would be so fundamentally different, there just haven't been enough court cases yet.
When you buy something, be it a car or a computer, it should be yours to tinker with as you please. Manufacturer lock downs should be as unwelcome in cars as they are in computers or tablets. Of course, if you do change something, and you hurt or kill someone as a result, you better be prepared to accept the consequences.