Dude, half the time it's made with Cheese Whizz IN PHILLY!
So you can't get all cono-sewer about it. Although, truthfully, the other half of the time it's made with provolone sauce... which is... excellent.
Besides, this is a pizza, so everything is allowed (except some things).
No, "everything" is NOT "allowed" on a pizza ( well it shouldn't be and all of the stupid, bizarre toppings on pizza, started in California, by people who don't know anything a bout food ! ); then it is something else.