As a practical matter, how could opt in work? Suppose you've opted in and I haven't. We both walk into the pizza parlor. How does the system know that you've opted in without attempting to recognize both of us?
For example, if an unrecognized face was logged on 47th street, then an unrecognized face was logged on 50th, then 52nd, then 55th (where a crime occurred), then back down to 47th, you might be able to draw some patterns about where to look for your suspect even if they hadn't opted in.
Edit: Also, you are trusting that the system isn't storing markers for people it doesn't recognize. My concern would be that if I opt in later, suddenly all my past history becomes discoverable. That's a lot of trust to put in the hands of private firms (or the police), neither of which I'm comfortable holding that information.
In fact if I were a not-quite ethical user of facerec, I'd flag and bin all vectors and image chips that failed to ID.
I work on facerec and ReID. But it's for deep fake detection and the like, blue team stuff. But the deep ID technologies are very concerning to me in general.
Given how prevalent the tracking of users is, anonymous or otherwise, I'd be extremely pessimistic about this assumption.