Because i heard that Kindle is allowed only ebooks for amazon ebooks store. (dealsagar.com)
I read a lot of amazon ebooks, and this field is just packed with self publishers, and usually the cover will give it away, but once in a while you come across one where everything is just right. (writerunboxed.com)
So unless more / all eReaders allow this, reading Amazons eBooks on non-Amazon devices is nigh impossible. (goodereader.com)