You own the hardware. But in the modern world we do not own the software that allows us to make use of the hardware. If deactivating OnStar requires a modification to the car's software, I can definitely see how it could effectively be illegal to deactivate. In the world of modern tech, the concept of ownership is practically dead.
But I don't know if a modification to the software is actually necessary or if that's what OP was getting at. Just speculating.
But I don't know if a modification to the software is actually necessary or if that's what OP was getting at. Just speculating.