I'm originally from Biloxi, Ms. The tradition started at Hugo's restraunt on Division street in Biloxi. Everyone went there for Pizza. The tradition grew. When Domino's came to town they learned that they had to provide small packs of catalina dressing with their pizzas on request. Even in Pizzahut there are bottles of French or Catalina dressing on the tables.
I know live in Slidell, La. Nobody in Slidell does it. People look at my family and say "Hey, you must be from Biloxi" when they see us put french dressing on our Pizzas.