I think you are missing the point; perhaps there are bunch of kids just fiddling with their ECU on an otherwise stock car where you live.
In my experience though, tuning is mostly done in conjunction with changing other parts (headers, ports, cams, add a turbo, fuel kit etc.) so the manufacturers long consideration of the best balance for the stock engine is no longer relevant. You have to retune to get proper performance; fair that tuners are rarely considering emissions like the manufacturer would, but changing the airflow characteristics without changing the tune is a bad idea...
Where I live, cars need certification/a license to be operated. The manufacturer does that for you, so you can obtain the required document quite easily for your car.
Aftermarket parts are regulated - you can't install what you want, it needs to be certified for your specific make and model (and motor variant, etc.) - otherwise your car is not legal anymore.
This applies to chip tuning as well. Since you're modifying a certified part, it looses the certification - obviously.
What new cars are allowed to emit, is strictly regulated.
Further, since this very month, we have a carbon-based tax on new cars (based upon the aforementioned certification from the manufacturer). Thus if the emissions of car are changed, you're basically evading taxes as well.
Where I live, chip tuning is almost only done to get a few HP on the cheap. And it is unfortunately very popular and nobody really cares.
Edit: You can seek in individual certification for some modifications, added to your documentation, but a tampered with ECU is not one of them I think.
Oh as I just remembered one thing that I forgot to add in the other reply - ofc I agree with most of your reasoning here. But I think that (regarding emissions) we need to find a better way to test/confirm this. As I believe you should be able to modify stuff you own.
But I just remembered regarding certification - BMW is selling official performance/tuning kits (Sometimes called M-Performance tuning kit), where you get a whole new ECU with more power (and maybe some other parts, but nothing major for sure). And now I really wonder how is the legality of that. And also adds another point, that there are valid and reasonably safe tuning options.
Ok, we live in different places and have different experience.
Most places I have lived in north america you are relatively free to install whatever after market parts you want, including things you have fabricated or modified yourself and people do some pretty extensive builds. That just doesn't work without changing the tune also.
In my experience though, tuning is mostly done in conjunction with changing other parts (headers, ports, cams, add a turbo, fuel kit etc.) so the manufacturers long consideration of the best balance for the stock engine is no longer relevant. You have to retune to get proper performance; fair that tuners are rarely considering emissions like the manufacturer would, but changing the airflow characteristics without changing the tune is a bad idea...