There are huge threads on FlyerTalk for each airline fee credit, detailing exactly does and does not trigger the credit for each airline. As you can see from reading those threads, they occasionally tighten up the criteria, but more often than not let loopholes persist for years. And these are loopholes used by hundreds of thousands Amex users in the US.
You have to remember that, for Amex to ensure the Qantas credit is only triggered by flights leaving the US, it would need to code all that into their system.
To what end? To prevent 20 Australians using US Amexes to buy flights in Australia in USD (taking a 5% haircut on the exchange rate conversion in the process).
They're going to lose more money in programmer pay than what they're going to save in cracking down on this loophole.
Am I saying the loophole definitely still works? Of course not. But you have to remember the incentives at play here. 99% of people making use of the Qantas Amex offer will be Americans buying flights to Australia.
Moreover, the offer is largely paid for by the retailer, not by Amex. So it is Qantas that loses out, not Amex. Even less incentive for Amex to crack down on it. We know that retailers pay for these offers after the hilarious fail that was the recent Elegant Office offer:
(Update: Shipping Fee & Cancellations) AmEx Offers: The Elegant Office online, Spend $100 & Get $100 Back (Free Stuff) - Doctor Of Credit