Expectation of privacy is quite establish in law:
For instance, the South Dakota supreme court recently found that leaving a webcam on public property for months in order to record everyone who showed up to and left a private residence to be a fourth amendment violation.
https://www.criminallegalnews.org/news/2017/nov/16/south-dak...
In the same way a photo of the visible spectrum is not the same as a photo of lightwaves outside the visible spectrum, a photo is not the same thing as seeing someone, and using that photo to track their every facial expression, emotion, and interaction isn't the same as a photo.
Law has not yet caught up to technology, so we need to change the law.
An analogous example: There's no speed limit for walking/running because human beings cannot run fast enough for it to be a problem, but we had to come up with speed limits for cars because they could do much more damage due to how fast they could go.
Also, while anyone _can_ take a photo or video of you, they cannot do whatever they want _with_ that photo or video. If they use that photo to portray a scenario that simply did not happen then they can be in some serious trouble.
Facial recognition going mainstream will have large ramifications.
People don't expect privacy in public but they also don't expect all all their public movements to be recorded, stored, collated and analyzed.
These are 2 different things with dramatically different consequences. Any discussion on privacy needs to account for the distinction.
Just because a person can take my picture in public doesn't necessarily mean they should be able to use face recognition to find out my name and lookup other information about me.