How is a car's computer in this way any different than any other modification to the car? What's your point here? If I remove the brakes from my car or install them incorrectly, I may not be able to brake before hitting you. Does that mean it should be illegal to repair your own car? I really don't understand what your point is.
It's hard to introduce a bug into mechanical systems like brakes. Most home mechanics can tell immediately if they did something wrong.
It's hard not to introduce bugs into programs. How many "tinkerers" can debug and safety-check a complex embedded computing system that manages complex electrical and mechanical systems in real time?
