"If the cameras are scanning images and performing a check against a known database and discarding all the ones that don't match, that's fine with me."
But the matching isn't 100% correct. Some people will be flagged as being stalkers when they're not, and some stalkers won't be picked up by the system, aka false positives and false negatives.
So assuming fifty something concerts, with about 40,000 people at each, that's two million people (!), so even if their false positive rate is only 0.001%, that's still twenty people who were presumably thrown out by her security because the facial recognition didn't like their face.
Biting the hand that feeds IT
