I'll give you two explanations from the discussion of this article in the Italian subreddit: - asking for marinara with mozzarella is like asking for a vegetar

I'll give you two explanations from the discussion of this article in the Italian subreddit: - asking for marinara with mozzarella is like asking for a vegetarian dish with some meat on the top - it's against the very principle of a pizza with poor ingredients that were available on a ship (marinara = marine, a sailor's pizza) - Italian pizzerias usually have huge menus with dozens of combinations and there's really no need to ask for custom ingredients, but for that subset of places that do allow that, mozzarella is too expensive to be put in the category of "extra ingredient for half a euro", so a client asking for a marinara base instead of the more expensive margherita base (that already has the cheese), might be looking for a loophole to pay less.