I don't know anything about "rules", but it does come down to licensing.
As I understand it, EA holds the exclusive license for these three manufacturers, at least for the PS2 console. If a company wants to add the cars, they have to go through both the manufacturer and EA to do it. And EA is far too threatened by the GT series to allow PD to have the cars. They know damn well that if GT had those three manufacturers, the NFS series would become obsolete instantly.
The fault is partially on the manufacturers, but it's also in EA's court. They probably spent huge sums of money to lock up those licenses.
I remember reading somewhere that Ferrari actually wanted to have their cars in GT, but the license agreement with EA prevented it.