Lauren Dragan, writing for The Wirecutter:

The compensated-review process is simple: Businesses paid to create dummy accounts purchase products from Amazon and write four- and five-star reviews. Buying the product makes it tougher for Amazon to police the reviews, because the reviews are in fact based on verified purchases. The dummy accounts buy and review all sorts of things, and some of the more savvy pay-for-review sites even have their faux reviewers pepper in a few negative reviews of products made and sold by brands that aren’t clients to create a sense of “authenticity.”