Since - in the United States - it is legal for a police officer to simply tail a person in public (this has always been strange to me) without limitation, the courts now seem to believe that any information that could have potentially been collated by having a person followed in public, is legal for police to collate.
Of course, there is a simple way to test this logic:
Suppose there would be enough police officers to tail every single American as they go through their daily rounds to work, college, night clubs, etc. Suppose further that this policy were enacted. Would the result be a free society?