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.
I don't think this is true? As far as I can tell any time the recording is mentioned in a complaint at the police behaviour the camera was off due to [battery life|maintenance|other].