The Voyage isn't a good example of Amazon's interest in non-US markets as there was a patent issue associated with the haptic page turn sensors that kept it out of some markets.
More commonly Amazon (like many asian electronics companies) releases their new hardware in the home market, where they are best prepared to deal with unexpected showstoppers and once they are satisfied they open up foreign sales. In effect, Amazon's american customers serve as gamma testers.
With Oasis, a niche premium product, they may simply be sure there are no issues to worry about. It's not as if the firmware is all that different from the battle-tested paperwhite firmware. And at that price it would be hard for them to dramatically underestimate demand.
(Which they did, and badly, for the Echo and FireTV as well as the original Kindle.)