Amazon has gone on a crusade to get control of the fake reviewer problem.
Read the actual Amazon Review Policy Guidelines. To get to the most recent information, use the slicing tools at Google and Bing to narrow the results to the last month of posts.
Amazon is still allowing authors to offer free and discounted review copies but you cannot exchange a book for a review or influence the content of the review in any way. And the verified customer badge is only allowed for Amazon customers who have spent more than $50 (or equivalent outside the US).
The author community is sort of up in arms over the fact that Amazon is deleting product reviews. Basically they are activiely purging sites and reviewers en masse when they detect any questionable or unscrupulous review practices or manipulation.
You can read several great posts analyzing the details that authors are seeing and experiencing including an excellent discussion by author Derek Murphy at The Creative Indie and this one by Author Marketing Expert Penny Sansevieri
It takes special planning to avoid violating the Amazon prohibitions. There is some good advice available describing the best ways to go about getting more verified Amazon customer reviews Make sure you slice to the most current results and qualify the advice by verifying the authoreity and credentials of the contributor.
Another set of Presari searches that lead to some really helpful criteria for deciding how to market a book is this one:
Getting Reviews on Amazon Cheat Sheet
and this one