According to emails that author Steve Weddle has received from Amazon, authors cannot leave reviews for other authors because they are deemed to be in direct competition with each other.
But leaving nice reviews for a friend, as Weddle did, hardly seems like something a rival would do. The LA Times reports that the practice of writers penning nice reviews for fellow writers, as with blurbs on a book jacket, "can look a lot like back-scratching," but "it's not at all uncommon -- and it's never been discouraged" within the publishing industry.
When Weddle investigated the matter more deeply, Amazon sent him the following email:
"We do not allow reviews on behalf of a person or company with a financial interest in the product or a directly competing product. This includes authors, artists, publishers, manufacturers, or third-party merchants selling the product," Amazon repeated. The company added a new closing: "We have removed your reviews as they are in violation of our guidelines. We will not be able to go into further detail about our research."
h/t LA Times