Earlier quoted context omitted.
License plate scanners are actually just scanning everyone so there is no slippery slope involved. Right now we are all being watched. It's a pure cost issue; camera + OCR + database lookup is really cheap so just tag everyone.
How did we go from the FBI flying aircraft with video cameras to license plate scanners which scan 'everyone'? I ask because there is no mention of license plate scanners in any of the parent comments to this, or the article. Also while I assume by everyone you mean everyone who passes through the field of view of the scanner, and not actually everyone; phrasing things in false absolutes like that does not further yo…
Basically, aircraft follows car X, it passes by a scanner, you now know every car with timestamp that passed by the scanner and can look it up on the video. Doing this automatically is just a question of having enough video coverage and some fairly basic video analysis. They don’t even need to do this in real time as you can write the software after the fact.
In other words because they already have license plate scanners and are doing OCR + lookup they just need a video of the cars after that point.
Note: I doubt they have nearly enough footage to track everyone right now, but the cheaper it is to add one more camera and archive the footage the closer you get to that point.
PS: You can even back track cars so if you have a lot of footage of a car that then passes near a scanner you then know its path from wherever the footage starts.