Or in other words, Amazon is the ultimate middleman. It's not about ebooks. It's about making Amazon the most frictionless way to buy anything.
If they own the market then they can dictate royalties to publishers.
If they own the market, they can disrupt the publishers. If I were Amazon, I'd be creating software infrastructure to enable serious businesses to create and market curated collections of content because that's all publishing is in its purest form, be it books, music, or whatever. Distribution is plumbing, and why would someone choose to compete with Amazon on that front?