The point needs to be made that, absent probable cause or reasonable articulable suspicion, the police/government has no authority to track anyone. So instead of you and I "hoping" that they can't follow us without a good reason (and thus, by extension, "hoping" that they won't abuse the privilege), they are first required to have a good reason before being allowed to follow us.
I was referring to that case where a police officer follows someone out in the open, on public streets. In that case, the cops have as much freedom of movement as anyone else does. If they were to trespass on private property, or take any other action that would be illegal for a normal civilian to take (wiretaps, access to any non-public corporate data, tampering with someone vehicle to attach a tracking device, etc.) then yes, they should have to get a warrant.