I can drive down highways in most cities in the USA without my license plate being read (Flock isn't on highways). Also Flock as integrated mostly just records license plates. It's not recording video 24/7.
But while our surveillance is not as widespread as other developed nations, it is still quite commonplace. There are cameras everywhere and recording license plates seems like such a tiny and justifiable expansion.
People in the US also get angry at speed cameras or red light cameras, yet I personally think both are very rational things to want in busy areas!
Regardless, I’m happy to take a results oriented approach here. Does tracking license plates make it easier to catch criminals? Does it make it easier to track stolen vehicles? I suspect cities wouldn’t be signing these expensive contracts if they didn’t see any benefits.
And finally, surveillance of public spaces is not inherently at odds with personal freedoms. Your mobility is not restricted at all, your core rights have not been touched. And you are always welcome to go live in the woods off the grid.
I firmly believe that living in dense urban areas with millions of others requires a reasonably limited expectation of privacy in public spaces.