When you look at the reviewing history of some reviewers things click into place. Some write nothing but five star reviews for widely disparate genres. Jump from a cookbook to a thriller to a how to book. All the reviews being one of several in the reviewer's arsenal, and all written at around the same time. I initially bought some books based on a high number of rave reviews only to find them so poorly written that I couldn't finish them. Some of the "highlighted" success stories in Indie publishing seem to have quite a lot of this type of review. They are distinctly polarised, either all five star raves, or one star rubbishing and little in between.
On the last couple of Freelancer com jobs I noted that the person requesting the job insists on the reviewer downloading a copy for each review in order to have the "verified purchase" box ticked. To this end they state that they will offer the book free for an initial period, then increase the price. Others offer to reimburse the reviewer for each purchase as a part of the payment for the job.
It seems that Amazon is actually doing something about it, some of the reviewers are complaining of accounts being frozen.