> If you build a system you can query for any face and get the identity
True, but you can't. Facebook can't. That's (one of the reasons) they suggest tagging amongst your friends, not amongst all of FB people.
You can identify one person (with a certain degree of certainty) against a certain range of candidates.
Or you can search for one person on a small sample of images (100s or 1000s of images).
So yes you can have a db of "persons of interest" and that will be searchable when someone appears at an airport. But if the number of people in that DB increases you're going to have more false positives and lower confidence.