I think this is always true for any platform. With Apple you worry that your popular app might get so popular, Apple creates it themselves. In some ways, I think the point of platforms is to scale the vision beyond what you can do yourself.... until you can do it yourself.
The difference between the App Store and Amazon's model is that it's barely worth Apple cloning a competitors app because compared to their overall income, Apps will only ever be a rounding error.
Amazon Retail on the other hand, sell physical stuff. It's their bread and butter.
