It is not being done behind the restaurant's back. The restaurant is paying the customers (in free pizza) to leave the feedback.
1) The existence of a contractual relationship or beneficial business relationship between two parties. 2) Knowledge of that relationship by a third party. 3) Intent of the third party to induce a party to the relationship to breach the relationship. 4) Lack of any privilege on the part of the third party to induce such a breach. 5) The contractual relationship is breached. 6) Damage to the party against whom the breach occurs.
Yelp may have a case for tortious interference against the restaurant if they are paying customers to break their Yelp terms. Yelp still sucks.
http://www.troutmansanders.com/files/upload/Tortious_Interfe...
I would agree in this case Yelp would probably not be laughed out of court. However, that's a fairly low bar and IANAL.