Realistically speaking, a proper surveillance UAV is far outside of rifle range. One that would be used for airsoft would not be, but to simulate the "eyes in the sky" experience they should be considered out of bounds for firing at.
Very little that is troop carried would take one down in reality, but if some asshole wants to bring an airsoft Javelin to the field, if he can target it he should be able to "disable" it, at referee discretion. Of course, effectiveness relies completely on the cost of said "Javelin" vs UAV.
But that is just how I would handle it as a ref/organizer.
Age Verifier - Lower Vancouver Island
Brotherhood of Nod - Nod Prime || Vancouver Island Airsoft League - President