There's no mention of her telling the clerk that she was a U.S. citizen. She told him "I'm from Iran". What's he supposed to do in that case? Can't blame him for not taking the risk. That's a serious federal law (and it's not unheard of for the feds to run sting operations, when it comes down to that).
The other guy was definitely trying to buy the device for an Iranian citizen.
The Federal Law says nothing about selling to a person who has no intention of bringing the item back to Iran, but simply has either a Farsi accent or was born in Iran. There was no mention of an Apple policy that requires its employees to ask "What is your citizenship?", but instead automatically assume nationality based on the comment "I'm from Iran." If the onus is on the consumer to clearly state his citizenship, then shouldn't there be a sign that lists the legal conditions of purchase?
I am most bothered by the fact that the Apple rep recognized Farsi and inquired further. So is it pure coincidence that the Apple rep knows Farsi or is every Apple retailer trained to recognize Farsi? And even if the Apple rep recognizes Farsi and inquires further, what if the consumer lies and says "no, I'm speaking Hindi", then what? Polygraph test? Bring in an interpreter? Call security to escort them out?
However, it seems pretty clear that the Apple employee interpreted Sabet's words "I'm from Iran" as meaning "I am an Iranian citizen." This is a perfectly reasonable interpretation, and there's no indication that Sabet made any effort to communicate her citizenship.