I don't believe in everything in black and whites. Perhaps you collect data and save it for a fortnight and within that fortnight presumably any crimes which may have occurred within a certain area will be known and one might start looking for cars which were within the area. After a fortnight if nothing comes to pass, then presumably you really don't need the data from that locale. And the case might even be made for up to 6 months or a year since sometimes criminal investigations do take some time. Perhaps you make the case that within a fortnight an area needs to be declared a crime scene in order for the license plate collection data to be maintain for up to a year.
But then you save the data for the foreseeable future and who knows what happens with it. Sheriff Arpaio or some gung ho type like that decides computer programmers needed to be monitored and bam, every where I've been for years on end shows up on a map? That seems a little iffy even if the scenario seems far fetched right now (really far fetched since I don't have a license much less a car at this point in my life, but asides aside).