Didn't the recent ruling on the FBI using gps trackers in cars disagree with that? I'm pretty sure the entire ruling was that despite being in public a person has a reasonable expectation of the privacy of where they are traveling and so to track them would require a warrant.
There is a difference between being in public and being followed. You have no expectation of not being seen while in public, but you do have a reasonable expectation not to be followed everywhere you go, including into presumably non-public (i.e,. privately-owned) properties.
In the GPS case, the use of GPS trackers could include (and frequently did include) "following" a person into private property where the follower would not otherwise have permission to be, i.e.., the doctor's office, lawyer's office, etc.
