It'd be too easy for authors to set up an alternative Kindle store, for one thing.
Sure, and your customers can just walk across the street to that other store if you don't want to sell at Walmart.
Except they don't.
The trouble is not the ease of use. Most authors I know offer their goods at half a dozen e-stores. And for all the money that Apple, et al have poured into their bookstores, the customers aren't interested in anything but Amazon. In fact, Amazon is often used by the book stores as a whole to figure out what's being published.
For the *vast* majority of e-book readers, Amazon pretty much defines the book industry. And unfortunately I don't see that changing any time soon. (And if B&N collapses, I see its dominance extending from e-books to books as a whole.)