Wait, what? Last time I looked, there were about 6 million vehicles with OnStar. It's certainly on a lot of new cars, maybe even most, but all? In any case, there's a good argument that reading off the GPS data from such a device wirelessly is less instrusive than physically attaching a device to the vehicle.
I just don't agree that sticking a transponder on my car --- that I'm free to remove if I see it --- is really all that intrusive. It seems less intrusive than them staring through the windows of my car.