I think the tomato is not totally necessary. You can get a pizza with a white sauce (Bechamel?), ricotta, and garlic at some of the places around here. I don't think it is a weird American chain thing, if I remember the story correctly my mom first encountered it overseas...
"Pizza Bianca" is very common kind of pizza in Italy: no tomato, with mozzarella, sometimes ricotta; commonly offered with a variety of toppings, often prosciutto.
