Some publishers wanted to be able to offer their books where the text to speech feature would cost extra per book. Audio books and text books, though of the same material, are considered different presentations and have different copyrights.

No one really thinks Amazon could not have challenged and Amazon in no way gave on their correct belief that separate purchases would not have to be required under copyright law. Still, it is not in Amazon's interest to deny either publishers nor themselves a separate revenue channel.

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On a separate note, I've noticed that amazon is providing some book titles for the kindle free as part of a promotion.