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.)

Don't buy them. I don't see why you want to control what other people buy though.