I've seen it first hand. My friend is a veternarian who was working in a very expensive neighborhood in California. Every time he got a 5 star review, it wouldn't display on his Yelp page. A few 1 star reviews did appear though. Yelp would call him and tell him essentially the 5 stars would reappear if he advertised with them. He had no choice but to pay and they all appeared, driving his average back up. How would you propose to prove it? And what would proving accomplish since many folks already believe it?