This is based on the assumption that the car is only ever used on public roads. It's usually a safe assumption, but data is still being transmitted from private property whenever the car is parked.
Similarly, if your car is sitting in the driveway you have no reasonable expectation of privacy in your car--everyone can see it; everyone knows where it is. Presumably the police could not sneak into your locked garage and plant a tracker. Presumably.
The jurisprudence gets a bit tricky when you start talking about what's inside your house. See-through-wall heat guns, for example, are not allowed without a warrant because the home is the paramount protected place under the 4th Amendment.