Why stop at tracking license plates? Once you've started you should report on pedestrians, bikes, whether people are at-home, which building people are entering, the way they walk and many more. Those are all "alternative revenue streams" that are as valid from the operator/investor point of view and completely unrelated to whether the way of transportation or activity is meant to be untrackable or unrelated to a way of transportation at all.

There is also a factor of scale: a cop can follow you, but a system where everyone is monitored 24/7 is a very different story.