Amazon did not delete 900 reviews of Hillary's book. Amazon deleted 900 comments that weren't book reviews. Amazon reviews are for reviews, not comments.
Ok, that makes real question whether they do this as a matter of normal course or if this is a case of a politician "author" (ghostwritten of course) receiving special treatment.
And the answer is, unsurprisingly, that this is definitely not standard Amazon operating procedure.
Edit: I misread this comment, and agree that a review isn't a review if it's just a comment. Leaving my original comment for context of my error :-)
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No, they were reviews. See the screen shot in the article: it shows stars (in this case a one-star review) with the review commentary. Followed by:
"Except that in all likelihood the overwhelming number of bad reviews on Clinton’s book were not from genuine readers, but from a concerted attack to bring down the average reviews on her page. By 3:05pm New York time, Amazon had apparently deleted over 900 reviews of the book."
And the answer is, unsurprisingly, that this is definitely not standard Amazon operating procedure.