Whether or not the car is street legal is an entirely separate concern, and certainly not one that should be enforced by allowing the manufacturer to prohibit all software modifications of any sort.
It's like preventing customers from changing their own tires because "they might do it wrong and cause an accident". Yeah, they might. But it's not the car manufacturer's responsibility to prevent that, and it certainly isn't justification for locking down the car to only be drivable with manufacturer-installed tires, or remotely disabling cars that use tires from unapproved brands.
> But it's not the car manufacturer's responsibility to prevent that
Imagine someone installs a cheap third-party ADAS in a Tesla that causes a multi-car pile-up. Should we expect highway patrol officers be able to determine if some third-party software interfered with the normal operation of the car? And, even if Tesla Inc is legally in the clear because the customer did it, they are the #1 target for bad news headlines because they're 'the Apple of Cars' (at least in terms of how much hype they attract), so by the time they set the record straight they've already lost some good-will.
It's like preventing customers from changing their own tires because "they might do it wrong and cause an accident". Yeah, they might. But it's not the car manufacturer's responsibility to prevent that, and it certainly isn't justification for locking down the car to only be drivable with manufacturer-installed tires, or remotely disabling cars that use tires from unapproved brands.