I am a direct competitor of Yelp and I have to say that the 'reviews' theory just doesn't ring true with me. Firstly at Yelp's size the chances of something like that leaking are high and the legal and reputational consequences would be catastrophic. Secondly, there are far more effective and subtler ways of promoting the interests of their advertisers and Yelp are one amongst the less aggressive exponents of these in the industry.