Once the police started to record every interaction with the public, along with their existing habit of placing traffic cameras left and right, they acquired enough data to track people.
Trying to restrict the analysis of existing data is never going to work. The police can always point to some death that wouldn't have happened, if they had ran Flock's software on their surveillance footage.
And even if by some miracle you manage to forbid plate recognition, cross referencing, etc, every ambitious (or lazy) detective would start doing it on the down low with OSS software.
Why must it be this way?
Because we're a bunch of bitch-ass pansies unwilling to tell our fellow countrymen (and women) to shove it when they permit the use of such logic.
I don't care "how many children need to die" or whatever, the sum total of the affronts upon our freedom a is not worth it. What even is the point of caring about the children if we're giving them a totalitarian dump to inherit?