a) Simply print out the email where Yelp explicitly extorted them.
b) Record the phone call using Google Voice or whatever, where Yelp explicitly extorted them via the phone.
c) Save the hardcopy of the letter or fax where Yelp mailed them the extortion offer.
Those are the only ways Yelp communicates. It would be trivial for one of the millions of companies in Yelp's directory to blow the whistle with concrete evidence. Yelp, an extraordinarily valuable company, would be pretty much ruined overnight. Why would they run this risk to squeeze out a few extra bucks?