In the case of former FBI man James Comey and his book, A Higher Loyalty anyway. From AJ Dellinger at Gizmodo:
Reviewers hoping to share their thoughts on former FBI director James Comey’s new book A Higher Loyalty will have to have the conversation somewhere other than Amazon. The massive online retailer is limiting the ability to post reviews to those who purchased the book directly through its platform.
Deadline first pointed out the limitation, which is preventing anyone who isn’t a “verified buyer” from leaving feedback on Comey’s tell-all autobiography. Reviewers who did not purchase the book from Amazon and try to leave a review are greeted with a message that reads, “Sorry, we are not able to accept your review of this product.”
…
There has been some suggestion that Amazon is only restricting negative reviews on the book, but that doesn’t appear to be the case. Attempting to leave a five-star review is greeted with the same message about reviews being limited. More.
It may sound harsh but the restriction is not without reason. Here at Uncommon Descent, we call such people noviewers.
These social pests drive by books they want to discourage others from reading by leaving negative reviews that sound suspiciously like the reviewer hasn’t read the book but knows he wouldn’t like it. And you shouldn’t either. But who can prove that? Anyway, if he hates the book enough to buy it in order to trash it, we are on the road back to civilization.
It is, however, odd that almost of the reviews of Comey’s book are currently five stars. The new policy is a file to keep an eye on in any event.
See also: What do we call people who refuse to read books they are attacking