Until recently, Amazon's review system allowed for a deceptively biased loophole: Users could receive free products from companies in exchange for an "honest" review. These were still pretty biased, so Amazon's getting rid of them. Photo by Zhao.
Starting immediately, Amazon will no longer allow reviews from customers who have received free products in exchange for reviews, no matter how they're marked. There is one key exception, however: The Amazon Vine program. With Vine, Amazon facilitates the review process directly which, at least according to the company, helps eliminate bias since Amazon won't deliberately pick out reviewers that give good reviews. Likewise, reviewers are less likely to worry their free ride will be cut off if they give a bad review to an Amazon Vine product. At least, that's the idea.
The other major exception is books. Publishers are still allowed to send advance copies to reviewers, as they have since long before Amazon. Outside of books, however, you should see fewer of these incentivised reviews in the future. As for the ones that are already there, though, Amazon says they will only remove existing reviews if they are "excessive" or don't comply with existing policy. So pay attention to the dates on those reviews.