The insidious thing about buying reviews is that the influence of those reviews on readers isn't the point of the exercise.
The issue is that large numbers of reviews and high star rankings figure (or used to figure) into Amazon's mechanism for deciding which books to recommend. If your book is featured prominently on Amazon, a lot of people see your book. Seeing your book is a precondition to buying it, and getting people to see your book is a major hurdle for authors to get over in an Amazon ecosystem that includes over 7 million books.
Amazon's various systems are designed to automatically recognize trends and memes and whatever else generates heat behind a book, and to get that book in front of its audience when it starts blowing up. It will move books into "recommended for you" lists on the front page, and it will add the books into the "customers who bought this also bought" lists more easily. It will even e-mail customers about the book.
Amazon is designed to identify whatever is popular or becoming popular and makes it more popular by essentially providing a huge surge of free advertising through prominent positioning on Amazon's site. So a very good way to actually become popular is to trick Amazon's computers into thinking you're popular already.
John Locke was able to do that because he spent a lot of money to contract for paid reviews in huge volume when nobody else was doing that. Now, fake reviews are very cheap, but much less valuable because they will no longer trigger Amazon to do all the things that Locke's fake reviews did for his book.
At the time, Amazon wasn't looking for authors to orchestrate hundreds of fake reviews, so Locke's fake review activity looked like an emerging bestseller to the algorithms, and they started promoting his books. Even at the $65 per review he evidently paid, according to the article, this was probably much less expensive per-pageview or per-click than Google ads or Facebook ads.
There are lots of other similar shenanigans that people have tried. For example: paypalling hundreds of people $2 each in exchange for buying a $0.99 e-book on Amazon at a specific time to engineer a ranking spike.