Earlier quoted context omitted.
Why would it fall apart? The plain view doctrine holds that the police don't need a warrant to observe things in plain view. Like your car driving on the public roads. Privacy laws are generally about protecting what you do in private , not in public. In the US, anyone can film you in public, government or otherwise.
It hasn't been articulated in court. But what flock has essentially done is created a GPS system without satilites. See Jones vs USA. Sufficiently saturated ALPRs across the country give you the same capability of GPS. Without GPS.
Jones vs USA involved physically interacting with a person's vehicle. Furthermore, the device would track users even if they were on private property.
By comparison Flock only operates in public, in places that are in plain view.