Well, somebody needs to play Devil's Advocate here, so I will. What if onboard vehicle computers truthfully are (or soon will become) so complicated - and so integral to the functioning of the vehicle - that an untrained hobbyist screwing with it could cause injury or death?
As if that were not already true for the cars before they used computers!
There is no more reason to have such a law now, that there was before.
Besides, read the report on the Toyota car computer system that was locking the throttle. I myself could do better than that!
(Of course, I'm an engineer... )