Jason Kottke covered this at a higher level, where consumers are taking into account the format of the media and not just the content in their reviews (unlike your average critic working for big media).

http://kottke.org/10/03/the-new-rules-for-reviewing-media

They're leveraging what power they have from available tools (star ratings and reviews on popular commerce sites) to put pressure on the publishers.

Is it odd that Amazon doesn't denote (or possibly separate out) reviews from users that it knows purchased the book (or DVD, etc.)? Privacy concerns? The user is already expressing an opinion on something the have [ideally] consumed, thus tying it to their profile.