the alternatives do not include a nationwide search as part of their features
Yeah I think people would feel much better if the cameras were operated by the local police department, and they were the only ones that had access to it.
ALPR (automatic license plate recognition) cameras have been around for a long time, but it previously wasn't an issue because access to the data was restricted. Same with security CCTV cameras. Other jurisdictions had to subpoena the data rather than being able to instantly access everything.
Flock's cameras are much more than ALPR though. They make a profile of every car they see, and can track cars even if they don't have license plates based on identifying features (dents, big scratches, bumper stickers, etc).