This seems to be Amazon's main unspoken strategy behind promoting positive reviews... many people want to be helpful and provide a review of a product that they purchased. That's great, but the problem is that most people are thoroughly unqualified to review things: "I took it out of the box and turned it on, five stars!"
(And I'm not even going to mention Vine Voice or whatever it is called, these are reviews you should ALWAYS ignore because they are given free products in exchange for ostensibly unbiased but implicitly biased reviews.)