I'm totally open to being convinced, but I'm skeptical right now that they are in fact that shady. I'm more inclined to believe that it's a placebo effect from suspicious people suddenly paying more attention to ratings after a sales call, and also attributing any negative dip in rating to the phone call they recently had. There are also third-party groups who call claiming to be able to list your business higher or rate it better, and I'm suspicious that some people confound these callers as yelp representatives.
And linked from elsewhere in the thread: http://www.latimes.com/business/la-fi-yelp-ratings-20140905-... -- it just seems like with no scientific study backing the claims and case after case being lost, I think the onus of proof is on the people accusing yelp of these threats.