The history of pizza as we know it today has no real ancient origin. It's actual beginnings leaned more sweet than savory. A flatbread w/ dates and cheese was found in numerous Mediterranean countries going back to the Roman Empire and before.. It wasn't until the Spaniards brought tomatoes back to Europe from the Americas before it became the modern pizza associated w/ Italian street food. Even then it was still very plain compared to today. More like flavored bread than anything. It took American GI's stationed in Italy in WWII to come home and open pizzerias in the US to really bring pizza into the 19th cent. So....if tomatoes, which originate in the Americas, are good enough for pizza, then pineapple, also a product of the Americas, should be as well.
In fact, the origins of foods most commonly associated w/ Italian cuisine (by non Italians) are either Chinese or American.
Afterall, Italy was the world's capitol of international trade for upwards of 1000 years, so it should be no surprise. I also don't see any reason why some get offended by the continuation of culinary invention/expansion. Especially in this day and age of overnight global shipping. I'm all for regional foods staying regional (coastal seafood, inland meats, northern cured/smoked, southern fruits etc..)...but pizza? Unless you are IN Naples...or NYC, or Chicago, it's anything goes! Hell, I've even once tried escargot on pizza at an Italian restaurant in a German speaking region of France. And that was in the early-mid 70's!