Disclaimer: I am not a lawyer, and this is not intended as legal advice.
As I understand it, that situation is peculiar to Australia and doesn't apply to most geographic restrictions anyway.
Amazon.com or any other ebook site is legally allowed to sell to almost anyone anywhere in the world (Cuba and North Korea being notable exceptions). The general problem is contractual.
Amazon's sales contract basically states that they can only sell an ebook to people in territories where the publisher has the rights to distribute that book. They can still sell to those people in general, but they have signed a contract saying they won't sell these products to them. That's why you can still order a pbook from Amazon to Australia even when you can't buy the Kindle edition of the same book from the same publisher. The contracts allow one but not the other.
It's the contracts that are the killer here.