In the case of automated multi-ton death machines moving at high-speed near other people, that seems totally reasonable. You shouldn't be allowed to tinker with your automated car and risk everyone else's lives.
As long as it's advertised that way upfront, then I don't see why not. Tesla is not going to be the only automated car manufacturer. Today, you can buy cars that only take specialty parts. You can also buy cars that take standard parts. I'm sure it will be that way with automated cars too. Same way that you should be able to buy an iPhone with all of its non-standard quirks.
As for safety, it depends on the specific parts. An automated car is a complete integrated system. Once people start swapping parts, the test-matrix to ensure safety explodes. People modifying the driving software in their own cars, for example, is definitely a no-go. (And modifying any software at all should be prohibited unless the driving and entertainment computers are completely air-gapped.)
What about the brake pads in my multi-ton death machine? Should I be prohibited from replacing them with third-party parts for the fear of risking everyone else's lives?
The big three made the same dumb arguments a hundred years ago... third party parts have never been a problem. People fought hard for the right to get replacement parts, and it's a shame we need to re-fight this battle because of the lemmings among us.
