I don't think Yelp's practices constitute a crime in the USA. The crime of a hypothetical Mafia is when they threaten violence for not paying. Yelp just changes what their site says about your business, which isn't a crime unless you can prove slander (very difficult).
Spreading falsehoods is libel. Pretending they can't know the truth doesn't shield them from culpability. Especially when their entire business depends on suppressing negative reviews.
