Yelp deletes legal (factual) unfavorable reviews for businesses that pay for Yelp listings. They allow illegal (slander and unfactual) reviews for businesses that do not pay to be listed.
I once left a great review for a long time favorite sandwich shop after seeing strange posts about the owners being on drugs. My review was simply about the food, and I was accused of knowing the owners.
Yelp deleted numerous legitimate negative reviews for local handyman in which several women reported bizarre threatening behaviors. I was surprised to see I was not the only one, when I saw Yelp reviews for him that another site cached and displayed. People should know!