Looking at that photo, if I were the one charged with a politicians security, I’d ask, “Hasn’t anyone here heard of rifles? You guys have great protection against an attack from within 50 yards. What about 200 or 500 yards out?”
During scheduled events they do sweeps with advanced teams, go in early and leave folks in place along the route. On roof tops etc, windows etc. If this were unscheduled and brief, someone waiting with a rifle at the ready at 200 yards out at an unscheduled event or stop, is not very likely to occur.