I mean, I kinda get where the Italians are coming from with this one. Dishes have a "soul," if you will, in that you can vary the ingredients but still get to something people can recognize as that dish. "Hamburger" is minced meat between bread, for instance, but within that range you can still create infinite varieties that can be called "hamburger."
It sounds like pizza marinara's very definition involves not having mozzarella... So asking for marinara with mozzarella is probably asking for something which, in essence, isn't marinara. Imagine someone asking for "hamburger in a hot dog bun"... We'd probably tell him "OK, but that's a hot dog, not a hamburger."
I mean, I kinda get where the Italians are coming from with this one. Dishes have a "soul," if you will, in that you can vary the ingredients but still get to something people can recognize as that dish. "Hamburger" is minced meat between bread, for instance, but within that range you can still create infinite varieties that can be called "hamburger."
It sounds like pizza marinara's very definition involves not having mozzarella... So asking for marinara with mozzarella is probably asking for something which, in essence, isn't marinara. Imagine someone asking for "hamburger in a hot dog bun"... We'd probably tell him "OK, but that's a hot dog, not a hamburger."
Really, I don't think it's just an Italian thing.