1. I have no idea what obligations Yelp has, legally or otherwise.

2. Legal definition of extortion (IANAL): The use, or the express or implicit threat of the use, of violence or other criminal means to cause harm to person, reputation, or property as a means to obtain property from someone else with his consent.

Seems like (to my untrained and uneducated self) that this would be Yelp damaging the Company's reputation because the Company refused to pay Yelp.