If someone likes pineapple pizza, they should be allowed to enjoy it.
Please feel free to enjoy whatever suits you (I know you don't need my or anyone else's permissions!).
And in the same way, please allow me to state that:
The combination of sweet + slightly acidic taste of the pineapple with the tomato and mozzarella makes it disgusting*.
Of course, this is just my opinion:
I won't stop anyone from putting pineapple on top of a good pizza and then eating it.
I might try to dissuade them though, as I would do if someone came to me with a nice 18 k$ scope, a Dremel and a bunch of RGB LEDs.
In both cases I would remain convinced of their bad taste.
But it's true, we Italians are quite protective of our food heritage.
Put parmigiano on a fish based pasta, and you'll learn some 'interesting' words, at the same time some regional fish recipes use cheese, e.g. Orate alla pugliese (pecorino romano, even stronger than parmigiano).

* In general I like mixing sweet, sour and other contrasting tastes. E.g. I'm a fan of Lingua in dolcefforte. (Sweet and strong tongue: ox tongue cooked with dark chocolate and vinegar). I'll try anything that's considered edible, and I like mostly everything.