This is a tough issue.
On the one hand, what about undercover cops?
On the other, it's known that the framers of the Constitution meant for the people to have access to arms partly as a hedge against governmental use of arms against the people. If facial recognition is a tool that the government will use against the people, then the framers clearly envisioned restricting the government's ability to prevent people from using facial recognition.
By default I tend to side with the Constitutional order, so I favor the people having unrestricted access to facial recognition tech. That said, I do see the chaos facial rec will bring. Not only for police, but frankly, for everyone else as well. Better start finding a side door out of that hotel tryst with your secretary fellas. Hypothetically, the wife could use some popular new facial rec service that runs against all the public photo or video upload sites for the day. You could be busted if you go out the front door of the hotel just when some college girl is snapping a selfie to share with the world. Or some dad thinks sharing a picture of his son's football team, taken at just that instant, would be really cool. All of which show you and your secretary exiting the hotel in the background.