Makes sense to me. If sticking a GPS transmitter on your car violates a fundamental right to privacy, surreptitiously following your car with a combination of cars, closed-circuit cameras, helicopters, and whatever-the-hell else they use must violate that same right. Yet clearly the police can surveille whoever they want without a warrant.
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.
But your private property isn't necessarily protected. For example, police don't need a warrant to fly a helicopter 50 feet above your fenced in back yard while they're searching for marijuana plants. The court says they're in the common airspace and you have no reasonable expectation of privacy in the common airspace.
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.
