No, you're missing the point.
There is no logical reason for Apple to be making 30% off Amazon's books.
you are absolutely correct.
Amazon, and competition has the right to go sell their books elsewhere then.
there is no monopoly in tablet / phone marketplace. There is absolutely nothing that ties Amazon to require sales on iOS devices. Amazon has chosen to do so because they believe it will help reach more sales. Apple has in turn effectively said "fine, you want to use our platform to reach more sales, taking advantage of the ecosystem and market size we have, then pay us for it"
I think Apple is entirely within their right.
I dont think that in the long term Apples business plan here is smart, as what happens to Apples Marketshare of iOS devices if lots of companies start thinking it's no longer in their best interest to develop for iOS? here's an example: See RIM.
They are still within their right to do so though. And the vendors are within their right to take their business to a competing platform (which depending on the different charts, have larger market share these days)