I think the risk to the carmaker is your customers getting used to the apple infotainment system. Maybe they start to see the car as less a Fiat and more an Apple car. Then the exclusive expires or needs to be renegotiated.
People forget that when the iPhone launched, carriers had an iron grip on their customers, to the extent of "pay us to put ringtones on our device that temporarily happens to be in your hands."
iPhone-in-car would have let Apple dangle some interesting data deals in front of car manufacturers, while retaining ultimate control, before the car manufacturers realized data was a monetizable revenue stream.