As Nagytow said, it is generally better not to buy games off Amazon Digital unless you're either in the US or they can be activated/redeemed somewhere else (Steam/Origin generally) since they won't provide support for non-US customers (Not that they should, since they don't really sell software outside US, so if you cheat it should be at your own risk).
A lot of people, myself included, buy games at Amazon US. I bet they know about it and allow it as they build their reputation as Steam competitor. However I think they might have some legal limitations. Some of my friends contacted Amazon support with various problems/question and their accounts got blocked and Amazon asked them for a proof that they are US residents.
I buy only Steam/Origin keys there, just to be safe. But even with Steam you need to be careful. Some of the keys are region locked. For example Civilization V and Mafia 2 can't be activated in Poland.
Amazon has no interest in limiting sales to the US. But they have to fulfill their legal obligations which are layed down in their distribution contract. Therefore, imo, they doing the least they can get away with (checking the US adress) without incurring any legal liability.
Therefore, it is best to let sleeping dogs sleep and don't bug support with obvious out of the US questions.