Ah, but following Asimov, the "Three Laws" may still apply even with the US drones. It all comes down to what you describe as 'human' when you state:
"A robot may no harm a human, nor, through inaction, allow a human to come to harm."
One of the books (Possibly 'Robots of Dawn') made it clear that visitors from Earth were not considered 'human', so outside the protection of the three laws.