Now we are talking sale of physical items on Amazon. Amazon is popular, doesn’t mean it has no competition. I can easily buy things on eBay, or Walmart or any number of big chain stores. Amazon isn’t the most popular online store where I live, eBay is. Kindle has lock in aswell, side loading is not convenient. most people wouldn’t know what it is. There are however many alternative ebook stores and many people do sell directly from a personal website cheaper.
Many companies do sell their products on their own sites, Music can can be bought from alternative websites. Unlike the App Store, there is no rule that says these items must be sold from a single source.
I do not understand where you're going with all this? Are these continually changing arguments meant to justify Apple locking out competition that allows them to freely set their own pricing? That no one would use alternatives anyway? That it's no different from retail?