I obviously have no training in this regard, but surely the snipers (whether USSS or local PD) are not ever pointing their rifles directly at whomever they're protecting, right? That's not only extremely dangerous, but from a practical/procedural standpoint, it would allow for immediate recognition of a threat because the only gun ever pointed in the direction of the Principal would be a person representing a threat. I could understand not taking a shot if you weren't positive they were holding a rifle, like maybe it's a really long lens on a camera or something, but if you know it's a rifle I don't believe for one second that they aren't authorized to take the shot if both the sniper and their spotter are in agreement.
Arguing otherwise means someone could be walking directly at the President, pointing a gun at him, and the USSS wouldn't do anything other than shield the President and wait for shots to be fired before neutralizing the threat.