Not Hawaiian, not Italian — Canadian
One fact defuses half of this argument before it starts: pineapple pizza was never invented in Italy, and it was not invented in Hawaii either. It was created in 1962 in Chatham, a small town in Ontario, Canada, by a Greek immigrant named Sam Panopoulos. He opened a tin of pineapple, put some on a pizza to see what would happen, and named the result after the brand on the tin: Hawaiian. He died in 2017, fully aware that he had started something far larger than himself.
That settles a common objection — "it is not a real Italian pizza". Correct, and nobody ever claimed it was. Faulting a Hawaiian for not being Neapolitan is like faulting a burrito for not being a taco.
February 2017: a head of state weighs in
Asked by a group of teenagers, the president of Iceland, Guðni Th. Jóhannesson, said that if he had the power, he would ban pineapple on pizza. The remark travelled the world within two days. His follow-up statement was a small masterpiece of restraint: he does not have that power, does not want it, and pointed out that presidents should not decide what people put on their food. The episode remains the best summary of this debate — everyone holds a firm view, nobody seriously wants a law.
Why it works, chemically
The combination that scandalises people is not aberrant at all. Pineapple brings sugar and acidity, both of which cut through the fat of melted cheese and the salt of cured pork — precisely the mechanism behind melon and prosciutto, sweet-and-sour pork across South-East Asia, or a glazed ham. Almost everyone who rejects pineapple on pizza happily eats at least one of those. So it is not sweet-and-savoury that bothers them: it is sweet-and-savoury in that particular place.
A split that does not follow the map
This is the one match on this site with no obvious geography to it. Convinced defenders turn up in countries with nothing culinary in common, and Italy itself is less unanimous than its reputation suggests. That is exactly what makes it the right ground for international comparison: once enough voices have been cast, the "your country vs the world" panel on this page will show whether your country is as decided as you think it is.
One thing is certain: of every debate open here, this is the one where the crime side shouts loudest, and the one where the delight side votes most quietly. The question is which of those two behaviours counts for more in a counter.
Sources: interviews given by Sam Panopoulos to the Canadian press (CBC, 2015-2017); statement and clarification by the president of Iceland, February 2017.