You're right that there is no real expectation of privacy when you're in public but I think the difference between tracking/storing movements and seeing your neighbor at the bank is you expect the loss of privacy to end when you get home.

The data something like this collects is so vast that it actually encroaches into the parts of your life you would reasonably expect to be private. For instance, you could determine that I'm seeing a psychiatrist, that I'm cheating on my wife or that I have cancer. That's private information you don't expect to be exposed by simply driving around.

To push the limit of the broad interpretation of your public data, consider the following scenario. Imagine in the not-to-distant future that a technology exists that allows people to scan, from a distance, the electromagnetic field produced by your brain and use that data to map your thoughts and memories. Would me simply stepping into public be a tacit agreement between me and the government to forfeit that privacy?