Why can't I buy ebook X outside of the US? Revisiting the territory question.
The answer is pretty simple.
Basically, when I sign a contract with a publisher, they acquire certain territorial rights. This is distinct from my copyrights, which are always mine and never sold. DAW owns the World rights for Toby and InCryptid. Orbit owns the World English rights for Newsflesh. DAW and Orbit may then sublicense these rights to other publishers in other regions (or territories), which is how you get things like Winterfluch and Feed: Viruszone (German editions of Rosemary and Rue and Feed, respectively).
The pieces I have sold to the Orbit Short Fiction Program ("Apocalypse Scenario #683" and "Countdown") were sold under a contract which, at present, covers only US territorial rights, which means that my publisher can't make those properties available outside the United States right now. They aren't allowed. And buying the rights for every possible market, in every possible region, is not always financially feasible with every work they publish.
It is also not always financially feasible for an author to sell all the rights to their work in every territory to the US publisher. Keeping World rights may mean a lower advance, but when I do retain those rights, I can ultimately earn more for them by selling them directly to foreign publishers. I want you to have and read my books in your preferred format, but I also want to pay my bills, and foreign rights sales enable me to do that reliably.
Orbit is working on making the short fiction pieces available outside the US; if you check the Short Fiction landing page, they note the problem exists, and that they're looking for a solution. Under my most recent contract with them, they now have the right to sell or license English language editions outside the US, which means that you'll hopefully be able to read it soon.
It's mildly annoying that it works this way, just like it sucks when I can't get the British or Australian TV shows I want on the right region format immediately. At the same time, this is how I keep the lights on, and how my publishers keep being able to do what they do.
ETA: This post has been pretty dramatically revised, following some clarification from smarter people than me. So if some of the comments seem to make no sense compared to the content of the entry, that's why. Sorry to confuse!