Observation: it is legal to listen to a conversation happening in public, and it is not typically legal to record it.

Some things that are not much of a problem at a small scale ("take a picture of a specific strange thing you see happening", "record one license plate of a specific car in relation to an incident") can become a problem at scale ("set up a video camera to constantly surveil the sidewalk and do facial recognition on it", "record every license plate that goes by and correlate your recordings with a million other people to generate a tracking map").

The problem is with pervasive surveillance, not discrete observation, and that's the spirit that laws about surveillance should attempt to uphold.