You don't have to outlaw homebrew vehicles. The car already has to pass an inspection to be allowed on the road. I guess, if it doesn't already, that the inspection will soon have to include that any safety critical software is unmodified. And when people are making their own fly by wire cars, and writing their own software - who knows how they'll certify it.

It's certainly true that the manufacturer should have no say over what you do with your car (if they had what would stop them from going the way of the printer industry and force you to only use their tires etc).

I might have misused the words "their" in this sentence so what I was trying to say is that since modifying software can very easy (no special tools, garage or knowledge required, just an unchecked download and a 5€ self made adapter of ebay) I'm in the favour of locking down security critical systems (the WHOLE car) as much as possible since not only you but everyone around you is concerned at least until checks associated with getting a valid license are updated and enforced.

What would be really great is if manufactures where to offer an API to allow developers and hackers customizations where they make sense (like adding your favourite online service) and not forcing them through the whole process of trial and error.