I don't see what difference there is in this and many other unobtrusive surveillance techniques law enforcement uses. Why not require them to get a warrant to just follow a guy around and photograph his activities? Lots of valuable intelligence is obtained by monitoring the activities of persons acquainted with the target who are not believed to be involved in anything illegal themselves.
The difference is that,perhaps,one cannot reasonably expect to enjoy "privacy" while being anywhere to which the general public has access.....like public streets,certain buildings,etc,etc.OTOH,I think that an individual has the right to expect that no government entity (particularly the police) will be allowed to attach a device to his/her private vehicle without either his/her permission or a warrant from a court.