It was allowed because before it used to work as intended but now it doesnt because its outdated since the auther stopped caring about said mod.
Well to be fair to it back when it worked I had the mod and it didnt entirely do that you could still put the vanilla parts on it. However with time its become totally broken thanks to as I said the developer abandoning it and not updating it.
I just wanted to post an issue with this mod that when I deactivate the mod as I have 700+ mods downloaded, it just sticks with the car and I tried to clear the cache but no success. I also tried to delete it but no success...