I could see them let it go on after a report - I’d expect they get a lot of threats and possibly fake reports too.
But for the rest… yea… as soon as he was sighted the security detail should have been on the move to cover trump.
Considering how people get shot for looking like they are thinking of a weapon when near the cops, it’s shocking that the counter snipers didn’t immediately shoot him once he aimed the gun at the officer.