It seems that we're all agreed that the current situation doesn't make much sense. As an employee of
The-Plot-Thickens (an ebook store specializing in fiction) I can confirm that the restriction is imposed on retailers by publishers.
I can understand why authors may want to release the same book to different territories via different publishers, but it seems to me that it's time that the online community is treated as a separate territory in its own right. In other words, a single publisher should be responsible for all ebooks across the world.
If you're dissatisfied with the way things are currently then I'm sure that the publishers will find your feedback useful. At the end of the day the customer drives the whole industry, so if you're struggling to buy their books then they need to know. Why not send them an email? You could even send the author an email. Most authors have their own website with a contact page.