Yes, it's essentially Amazon. But what can we (or anyone) do about it?
Not sure what the situation is today but, early on, we had to give up 70% of the list price if we wanted Amazon to sell our digital books. It's just mind-boggling.
And that's on top of the suboptimal user experience, because you can't sell a PDF via Amazon, and the Kindle toolchain is just awful. We have our publishing pipeline automated (DocBook to print-PDF, on-screen PDF, and EPUB); for a while we had bad Kindle/Mobi as well, but I don't think even that is possible today.
So, it's not only that Amazon gets a huge chunk of the money, you also have to accept the inferior quality of that particular distribution channel.