Bear in mind that most people cannot do this. You cannot open an account in another country merely for rewards arbitrage.
Isn't that what this thread is about?
In any event the end result is exactly the same as using a USA-issued card, with no foreign transaction fees, to make purchases in Euros.
That's true, but points can be earned in a number of ways. For example by referring new customers for a card. I recently earned 40k German MR points for signing up my father for a German Platinum card. He wouldn't have been eligible for a US card. Etc.
And finally, even if you simply use Global Transfer as intended by Amex, i.e. after you've moved from one country to another, it's a great way to maximize existing points from your previous country.