Amazon should introduce a progressive scale similar to the ones widely used in tax bracketing. The higher rates only kick in at price points that ensure the sale of a higher priced e-book will always result in more revenue for the author, even if the general trend is to take a bigger cut of higher priced items.
(Not that I am a fan of Amazon's publishing model - it just seems they are tripping over an already solved problem.)
For sure it’s an intentional strategy to keep books cheap. And indirectly also to prevent any competitors from selling the same books at a higher price.
When you own such a large portion of the ebook sales market, it very much could be considered predatory (bordering on monopolistic).
Larger authors are not compelled to agree to these terms sure, but if you’re a smaller publisher or author trying to get your work out there and noticed, it can be hard to gain ground without selling via Amazon.
As a "smaller author" who used Leanpub (not affiliated with them, just a satisfied customer – both reader and writer) to publish my Emacs Lisp book I can say that I don't care about putting it on Amazon, and I don't think I lost a lot (or anything) because of that.
OTOH, I published this book after about 7 years of weekly blogging, mostly about Emacs, so I think that everyone who might be interested in my book could easily know about it. And an intermediate Elisp textbook is rather a niche thing.
(Not that I am a fan of Amazon's publishing model - it just seems they are tripping over an already solved problem.)