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Amazon is making it impossible to remove the DRM from Kindle Books
(goodereader.com)
This is a most excellent place for technology news and articles.
They already ruled on this in favor of allowing you to back up what you already own. See video games, DVDs and CDs, video tapes, this is well established already.
They actually walked that back using blu-rays as an excuse. If there's any sort of DRM/encryption/etc, you're completely unallowed to circumvent it, even for personal backup.