It would be very hard to make CANBUS inaccessible from headlights, since that what controls it. However, the headlight shouldn't be able to tell the rest of the system that the key is in the car.
Logical compartmentalization like you suggest is a fine approach, but even better is to not allow physical access. Unless the car is in maintenance mode at the shop, the chassis should be sealed tight. Maybe the manufacturer decided to favor headlight maintainability over theft prevention, or was simply oblivious.