2 other data points from (disclosure) someone with ties to Yelp:

1. Ongoing independent research / A working HBS paper found no connection between advertising and filtering (see last page of: http://hvrd.me/ZNpuOy) [Why do we see advertisers are more likely to have their positive reviews filtered?]

2. A simple Google test like this: http://bit.ly/15QV1WS [Why are these advertisers not using their 'Delete negative review' button? Hint: because it doesn't exist. Which begs question -- If it doesn't exist, is there a rational incentive to sell it?]