[sourcejedi]

Unlike the Houghton Mifflin Harcourt editions of the Young Wizards ebooks presently available at Amazon.com and Barnes & Noble Online, which can only be purchased and downloaded by readers in North America, our ebook editions of the original YW novels can be purchased by readers in all other parts of the world.

Emphasis mine. The general rule is that publishing contracts require exclusive rights; there's nothing to suggest an exception here.

If it turned out the International versions were also being bought in the US, I'm sure she'd appreciate a private message. From someone other than the publishers' lawyers. (It has come to our attention...)

Some of you will have noticed that I’ve been overseeing a project whose time (I think) has come — international editions of the Young Wizards novels. The YW books’ North American publisher, Houghton Mifflin Harcourt, only holds publishing rights for the US and its possessions, Canada and (for whatever reason) the Philippines. When UK YW fans (who had print editions in the early 90′s) complained to me that they couldn’t purchase the books from Amazon in their ebook editions, I checked with my agent’s contract lady to make sure that self-published ebooks would be kosher, and then started to see what I needed to do about it.

...With the YW books I was forced into a situation that Smashwords could not be used to solve, due to the territorial restrictions attached to the works.