Self-interest and ingenuity combine to game almost any system, and Amazon's book review system is no different. Services exist to supply authors with paid reviews and hopefully bump them to a more visible position with Amazon's search algorithms. Amazon officially bans the practice, but it is difficult to see how such a ban could be effectively policed. On a more ad hoc basis, individual authors have been accused of ginning up support from family and friends and planting huge numbers of non-authentic reviews.
hat tip: Rhys Tate.