If Amazon sells 10,000 Acme Widgets and sends an email to just 1% of those customers asking for a review. How is Acme going to game that? Pay all 10,000 purchasers of their product so that the 1% that get the Golden Ticket to write a review give them 5 stars?
The problem seems to me to be that the reviewer gets to decide to post a review. Turn it around and have Amazon select reviewers at random and you've made it much harder to game.