The hiding of positive and negative reviews if you don't/do pay is hard to prove, but one thing incredibly shady thing that Yelp does and is proud of is: They

The hiding of positive and negative reviews if you don't/do pay is hard to prove, but one thing incredibly shady thing that Yelp does and is proud of is: They will prominently display ads containing positive reviews for competitors on a business's page unless that business pays Yelp.