KU benefits Amazon more than buying a book via Amazon, which can be 70% to author / publisher.
AFAIK all KU books are available to buy and not all ebooks on Amazon are in KU.
I read bought ebooks from anywhere except Apple or Barnes & Noble because they are far more a Walled garden than Amazon is, in reality. I read on a Sony PRS350, Mars, Libra and Elipsa. My three kindles are only used to test converted epub2s before upload to KDP (as epub2) and Smashwords (epub2 for local and distribtion, dual mobi for local) and Google (as epub2).
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