You don't even have to have actually READ the book in question to post a review on it, as can be seen in reviews of books by polarizing figures like Sarah Palin. Amazon seems to be trying to get around this with "verified purchaser" reviews, but they don't do anything to stop those reviews from going into the total rating.
And here's a problem that's happening more and more. Reviewers who have not actually read the book are posting one-star reviews solely because the book does not have a Kindle version. Authors usually have NO say about if there is a Kindle version, its price, and when it comes out.