But can you get Angry Birds from Google Play for your iPhone? No. Would it matter much if Angry Birds was cheaper on Google Play than on the app store? What part of one store (apps for iDevices) versus many stores (ebooks for iDevices) is too hard for you to understand?
Can you get an iBook from Amazon? No.
Developers and publishers are free to choose which markets to participate in.
Your "comparison" makes no sense.
Why should an interactive iBook containing colour pictures and a lot more content be restricted to the same price as a glorified, locked down, black and white PDF?
Apple offers a different product to Amazon, as different as the iOS Apps you use in your rather senseless example.