There is a very easy way around this issue, have the cameras centrally connected. Each day they download the current list of "valid" plates. If a valid plate is scanned, ignore it, that means no storage of time of scan, or location of the camera that did the scan etc... If however an "invalid" plate is scanned, upload all informaton about the plate.
If further investigation finds that the "invalid" plate is infact a "valid" plate then discard all info about the scan.
You could speed up the frequency of the database updates, but I doubt that it would improve the hit rate much, the only difference would be a priority update when a car is reported stoled or involved in some other crime...
There is no need to store data on people driving to work and home, picking up their kids and generally going about their daily lives. This is a massive invasion of privacy, and should not be accepted as "normal".
Disclaimer, I live in New Zealand, I hope my government hasn't slipped this far down the police state slope.