How exactly is that different from the usual publishers' practice of sending early copies of the book to a ton of reviewers and ask them to, well, review it in newspapers and magazines?
1) Professional critics are not usually the friends and acquaintances of the author. They have reputations and rules (enforced by their publication) that govern their behavior.
2) Amazon reviews are intended for customers who have paid for the book. Not for friends of the author that received it for free with the expectation that they would review it on Amazon.
It's Amazon's failing that they don't protect against this. If you did the same thing on HN the anti-voting ring software would kill the submission.
The vast majority of books on Amazon would have zero reviews if they only let you review a book after purchasing it on Amazon. Yes, it does suck that Amazon won't delete reviews even when the person admits they haven't read the book, or when they are just complaining that the Kindle version doesn't have a long enough free chapter. But other than that I have no problem with anyone writing a review as long as they've read it. Reading the book takes several hours, so the vast majority of people aren't going to sit there cranking out reviews just to get free books. There are people who do that, but it's really only a couple dozen on all of Amazon.
I think there is some assumption that Amazon reviews are representative of a large population of users, so if there are 10 great reviews and 5 good reviews and 1 bad review, it's representative of how a much larger population of users feels.
If you're writing a review commercially, with limited space, it only makes sense to write a review if:
* It is very positive and leads to purchases
* Negative reviews of popular/well known products (or, from big/well known companies -- a negative review of the Apple Cube is quite popular)
* It is part of a review of a class of products (either "top 10 devices of CES" or "all digicams compared")
There is no incentive to publish negative reviews of obscure products
2) Amazon reviews are intended for customers who have paid for the book. Not for friends of the author that received it for free with the expectation that they would review it on Amazon.
It's Amazon's failing that they don't protect against this. If you did the same thing on HN the anti-voting ring software would kill the submission.