We're not talking about a bootstrapped startup though; this is Amazon with all the legal expertise that means. They managed to evade £9 million in UK tax!
Just because Amazon is big doesn't mean that everything they do has to be big and full-featured right from the beginning. That's how big companies ossify.
I guess one could always find a justification. But the fact of the matter is that no other big US company managed to do what Apple did years ago: give money to non-US companies/individuals selling IP.
Google's AppStore still supports only a limited number of countries.
Amazon don't support sellers outside the US for any of their offerings (be it, selling books via Kindle Direct Publishing or selling apps in their Android store or selling EC2 images).
This is not a feature that's too early to talk about because AWS Marketplace is new. Supporting non-US sellers is something that Amazon just doesn't seem to be able to do it or doesn't care.
Frankly I don't know if it's a matter of red-tape (they would need a single EU presence, for example, to cater to the 27 member states) or just some kind of weird protectionism.
