Do you have any proof of that? Yelp is pretty well known for not accepting money or any other kind of incentive in order to show hidden reviews. It's all a search algorithm. If the person posting the review has no other reviews, no friends on yelp, no check ins, etc. it limits whether they will be shown. The reviews also can pop out of hidden status based on the reviewers other activity. One of my friends works for a small company with that issue. A ton of 5 star reviews but they are all hidden and only the 2-4 star reviews are showing. They tried to pay, tried to advertise on yelp, etc. None of it helped.
I used to have a bunch of my reviews hidden, but after building up my account and basically proving I'm a real person, I only have 1 hidden review.