Do you want to stick your head up over an wave and try to clamber up onto a roof while a guy a few feet away and slightly above you takes easy shots at your head? Or would you duck back below the wave and call for people in better positions to shoot, to take him out?
The real question is why there was no Secret Service personnel stationed on that roof.
My son as a Marine rifleman renewed his expert badge by shooting a perfect score at 500 yards with iron sights. As far as I am concerned that should be a minimum perimeter.