Not too long ago, I watched this video from ReviewMeta.com of how they used hard data to figure out that incentivized reviews are often biased and rated higher than reviews written by people who bought the product. No surprise here really since it's not exactly a secret that people can be manipulated in one particular direction by any number of hidden cues (in this case, given a free or highly discounted product).
For Amazon, this is a big problem because it basically means that their star-rating system can not be trusted. And if you've ever shopped on Amazon, you'll know that without their star ratings, Amazon is basically a garbage heap with no way to filter the good from the bad. So now, Amazon is officially banning incentivized reviews. Finally!