Two years ago I sat in on a call with a Yelp rep with a former employer of mine (small business connected to the real estate industry). The Yelp rep said "Advertising through Yelp will almost instantly increase you rating." My employer asked "Does that mean you'll manually fix my score?" To which the rep responded, "Your rating will get better if you advertise with us."
That was 2 years ago. I know Yelp has gotten flack since then, and maybe they cleaned up. But that stink of being a thug doesn't wash off quickly, especially in the small business community.