> I think there are two classes of software modifications: those that negatively impact the environment or other people and those that don't. The first is by its nature easy to detect from the behavior of the car, and I see no reason to regulate the second class.

Assuming they manifest themselves before a crash happens. When the software modification only impacts the ESP or ABS system in an emergency situation, it is too late. Only extreme thorough testing can make software fit for this kind of purpose. And experience shows, even then bugs stay unnoticed.