A rule I set many moons again is apparently still in place at Amazon.com. When I was catalog manager briefly (a six-month stint at the company in late 96 to early 97), I had to deal with the burgeoning number of user-submitted reviews which were posted live instantly on the site when I got there. I fought a battle over several months, culminating before I left, in which reviews were reviewed by a staffer before going live. I won this battle partly because more and more reviews were both unpleasant, offtopic, and not noticed until we got complaints. There were turds in the punchbowl, in other words.
One of the rules I set for my staff in reading over reviews was that reviews that attacked the author and that otherwise didn't address the substance of the book were off-limits. This is the "attack ideas, not authors" notion which is a specific case of a general forum policy on most forums prohibiting ad hominem attacks.
An Amazon spokesperson said in a New York Times piece (in Monday's paper), "the posts violated guidelines, which require reviews to focus on the content of the book, not the character of its author." The article covers a tax-denier's book that had reviews that said things like follow this book's tips and go to jail.