 Well, we need to distinguish between the law, what the law allows, and whatever their policy may be. Their policies could be completely inane. But what matters is the life of the president and the president to be elected. The law allows for them to use deadly force in defense of the president or any other innocent person the moment they reasonably perceive an imminent deadly force threat to that innocent person. That means they don't have to wait for a shot to be fired by the bad guy first. That means they don't have to be certain he's a bad guy. They could be wronged.