The problem isn't the license plate reader itself. In isolation, they're not the big brother utility. It's what happens when Flock decides to aggregate all the readers together to form a network.
If the license plate reader data stayed local (like they had for many years - they aren't new) you get far less problems. The problem was that the data started getting aggregated together to form a complete picture of people.
And it's not even useful - stolen car reports were proven to be rather ineffective where most of the alerts were useless because they were tags from other states with the same plate.
Jaguar has a huge problem with them, because of the odd New Jersey license plates that have letters, and two sets of numbers, one big, one small. The cameras didn't read the small numbers, so it results in the entire fleet of Jaguar cars they send out to reviewers cannot be used because one stolen car has a similar plate of letters and the big numbers.
The problem with Flock cameras is two fold. First, it's the network effect - they have cameras everywhere and they let anyone and everyone access them all as part of the subscription. The second problem is law enforcement is lazy and they do not consider that multiple states might issue license plates with the same numbers and digits, resulting in confrontations where a parent in an SUV gets pulled over because their plate matches a stolen motorcycle. Then there's the government issue where ICE is misusing the database to track people (or rather, ICE is misusing the AHJ to get them information to track people since ICE is not able to access the information directly).
It's a case of where if multiple jurisdictions nearby used different incompatible systems, things wouldn't be so controversial because it wouldn't be possible to track someone without needing access to multiple systems which would require multiple checks (e.g., warrants).
When the Flock network was small and limited itself to a few HOA roads no one really cared and the data that mattered was always accessed after the fact and no one could go beyond the cameras they had. Then they branched out and let cities along with private entities install cameras and gave access freely to the entire network.