They might not shoot to kill in cities, but they're damn sure going to shoot to kill at the Whitehouse. If a mob crosses that fence, they'll be mowed down. They only don't mow people down in individual scenario's, because security can generally handle them. The WH might be built like a fortress, but with a mob, no chances would be taken. They might have some men come out with weapons drawn to try to arrest them, but if they keep coming, there's no way they don't open fire. Part of the justification is that if they overrun security, they could get hold of weapons. If they get hold of military grade weapons, they become even more dangerous. Would not be allowed to happen. It would be no different than going for a cops gun.