I think it comes down to: Amazon and Netflix have enough money to pursue legal action against Apple. You don't. Hence, the rules that apply to you are different from the rules that apply to Amazon.

Remember what happened to Microsoft for bundling IE with their OS in order to put Netscape out of business? Making competitors' ebooks cost 30% more than your own ebooks on your own dominant platform can look a lot like that to the right eyes.

(Who would have ever thought that Apple would be worried about anti-trust suits!)