It’s against Amazon’s Terms of Service to incentivize anyone to leave a positive review. The product review ecosystem is one of Amazon’s greatest selling points - think about it, how many times do you go straight to the “Reviews from other customers” section of a product page when browsing products on Amazon?
Yes, Amazon has their Vine Voices reviewer program which is for Amazon’s most trusted reviewers. According to Amazon, “Amazon invites customers to become Vine Voices based on their reviewer rank, which is a reflection of the quality and helpfulness of their reviews as judged by other Amazon customers.”
Vine participants are not paid for their reviews, they only receive a free product as a sample for the purposes of the review. They are also not incentivized to leave only positive reviews. The Vine program is invite-only both for reviewers, and for the manufacturers of the products which are being reviewed.
So why do you occasionally see the phrase “I received this product in exchange for my honest review” in some product reviews on Amazon?
Since buyers place so much trust in reviews from other customers, it’s important for Third Party Sellers on Amazon to get reviews of their product. But very few customers actually take the time to leave reviews after they buy something. So there exists a catch-22 for Sellers:
No product reviews = no sales
No sales = no product reviews
To address this problem, some brand owners and third party Sellers on Amazon run promotions to give away samples of their new product in exchange for unbiased reviews.
Sellers must be very careful to ensure that they welcome both positive and negative reviews, otherwise they may be contravening the Amazon Terms of Service. It’s also against Amazon’s Terms of Service to try and manipulate the Sales Rank (the ranking of all products in) through the excessive use of free or discounted products in this manner (see: Prohibited Seller Activities).
*Note: this post was written in September 15, 2015 and a few things have changed since then. Sellers used to run promotions to give away samples of their new product in exchange for unbiased reviews, however this information is not valid anymore since it doesn't comply to Amazon rules. This update was made on March 1, 2018.