This kind of drones are relatively slow. The perimeter around in the president in any exposed situation is pretty large. Any appearance of unidentified flying objects on the scene is likely to trigger an evacuation of any key targets. For the kind of payloads small drones can carry, and with no possibility of changing the target, you don't even need to move the target very far for the strike to be non-lethal.

On the other hand, you could build a cruise missile (essentially a drone) that might be big and fast enough to hit your target. But those date back to WWII, so if they keep the Secret Service up at night, it's a pretty exhausted security detail by now.