Technically, this is not taking advantage of any public information. It is just capturing images of things happening in public. Anyone could set up a camera and capture license plates as they drive through an intersection.
So is it totally fine for a police force to scan license plates at every intersection?
The question is complicated, because the effect on people changes when you do something that is not worrisome on a small scale in an automated way at a large scale. When you add in that the people doing it have a large amount of power over people and it changes it even more.
I’d imagine it would be relatively easy to figure out where someone works and/or lives and just go find them.
Correlating across multiple public databases might make things even easier.
But I don’t think this is the point of the argument. The point is that those in power can gain more benefit from the information than the general population.
Let’s say all votes in elections become public. That information is more useful to a powerful political party than to the public at large, or a small unfunded group. If you don’t want that information abused, the argument goes, you should make sure it’s kept private.
The argument is that even if everybody has access to all information, only a small group is able to exploit that access effectively.
So, you still end up with a small group abusing their access to the information. You should therefore aim for privacy of sensitive information.