Why? If you don't like them simply don't use them, why limit things for those who do like to tune or modify them and why exclusively for these brands?
It's not that there's a law in real life banning modifying them, I get that these brands are something special in real life (at least they are to me) but in a videogame they're just another piece of data which these manufacturers agreed to when giving PD their license.
Wanting to keep certain cars stock is one thing, demanding or requesting everyone should is complete balderdash, especially since it involves taking away an option already implemented and being used.
I've got a few fully tuned Miuras on sport soft tyres and they're a hoot to drive and sound absolutely glorious, I've chosen colours which came close to existing Miuras and kept the fantastic looking standard wheels.
I must sound like a complete heritic to you and believe me if I ever got to own a real Miura I wouldn't modify a single thing but it's only a videogame aimed to provide fun and a tuned Miura to me is much more fun than the stock version (again in this videogame).