You're right. You know, if you haven't read the book, then the review you write won't make sense, especially in light of other reviews, which are likely to be more specific and corroborate each other in some way, regardless of the rating. I think this forces shoppers to be more critical thinkers and not expect everything they read to be true or as it seems. Now whether shoppers will actually BE critical thinkers is a whole other matter.
A hell of a lot less than reviews in magazines or on radio where the reviewers gets books free so incline towards a less harsh critique - same goes for Oprah, the only two books she recommended that I would second are "Pillars of the Earth' and "Middlesex". Unless it is at such a reduced price that it does not matter I never ever purchase a book without consulting the reviews on amazon. It has saved me a fortune in dead buys.