I'd love someone to point to evidence of this beyond pure anecdotes. Yelp clearly allows negative reviews on accounts which advertise with them which can be seen with this Google search [0].
Furthermore an independent Harvard Business School study showed no evidence of fraud [1].
Sure Yelp gives a hard sell, but until some disgruntled ex-employee blows the whistle, or multiple businesses record evidence of this extortion, I'm not ready to believe this is any widespread fraud. I'm sure there are isolated cases, but surely someone would have come forward to back up claims that Yelp can manipulate reviews and ratings?
[0] https://encrypted.google.com/search?q=site%3Ayelp.com+%22thi...
[1] https://www.yelpblog.com/wp-content/uploads/2013/12/hbs-stud...