Following complaints and a scathing exposé by Review Meta (previously) Amazon announced it will now ban incentivized reviews, a form of shill review written in exchange for free or reduced-cost products.
From here out, such reviews may only be written through Amazon Vine, according to Chee Chew, Amazon's Vice President of Customer Experience:
Today, we updated the community guidelines to prohibit incentivized reviews unless they are facilitated through the Amazon Vine program. We launched Vine several years ago to carefully facilitate these kinds of reviews and have been happy with feedback from customers and vendors. Here's how Vine works: Amazon – not the vendor or seller – identifies and invites trusted and helpful reviewers on Amazon to post opinions about new and pre-release products; we do not incentivize positive star ratings, attempt to influence the content of reviews, or even require a review to be written; and we limit the total number of Vine reviews that we display for each product.