Because it's a warrantless search. This is Virginia NOT Europe. Some people have left oppressive regimes where freedom of movement is extremely difficult, some just got sick of surveilance cameras everywhere.
The license plate scanner is just the first step, next will come 'security' cameras on street corners and the argument again will be 'if you do nothing wrong you have nothing to worry about'. Then what?
More cameras, GPS tracking on your cellphone, more checks when you go to buy a firearm, background checks to buy ammunition, limits on what you can buy, and everytime someone like you is going to chime in with 'if you have done nothing wrong why are you worrying?'.
This isn't about catching those who have done wrong, it's about protecting the freedoms Americans (especially Virginians) are born with, and immigrants have chosen to come here to enjoy.