Besides, people have been tuning their cars for 100 years now, you'd think that by now the police had some experience keeping the worst specimens of the public roadways.
The emissions (and taxes in the countries where taxes are a function of emissions) aspects are more likely to be the justification of any repercussions against these activities.
Couldn't the same case be made for doing your own mechanic work, though? We have braking, which is hydraulic and throttles which operate by wire. There's nothing preventing you from replacing the tried and true hydraulic brakes with something that runs on bluetooth. That would be just as dangerous as messing with the software of modern car brakes. More so, since one would hope modern car software has some failsafes built in[0].
[0]: they probably don't, but I know Sevcon does. Sevcon is a company that makes electric vehicle controllers for EV vehicles. They're popular with the DIY e-motorcycle crowd. Messing with the settings directly affects the power delivered to the motor.
Copy/pasting code from the internet is easy, so it has to be made artificially difficult. We live in a world where people put their iPhones in microwaves because they heard it would charge the battery. We have to keep these people very far away from the internal workings of their cars.
If you're curious, you should poke around http://endless-sphere.com/forums/.
Even with needing special hardware and software I'm not happy about it the tinkering. Cars are too much: 3,500 lbs of metal going 40 mph with 10 gallons of gas in it is a lot to go wrong.
I hate DRM, I really do. And I think we should have kids programming Arduino's and Raspberry Pi's by junior high, but part of me thinks they should bury the computers so deep in the engine bay you have to pull the engine to get to 'em. It's just my opinion, but if you don't have a (relevant) PE license, minimum, you shouldn't be allowed to mess with ECU's in cars that go on public roads.