As a SPAM junkie, I'd heard the Long Pig rumor before and knew it was debunked so I just did some digging and found this Straight Dope article
where the writer looked into the claim. It all stemmed from a quote in a book by Paul Theroux. From Happy Isles
It was a theory of mine that former cannibals of Oceania now feasted on Spam because Spam came the nearest to approximating the porky taste of human flesh. ‘Long pig’ as they called a cooked human being in much of Melanesia. It was a fact that the people-eaters of the Pacific had all evolved, or perhaps degenerated, into Spam-eaters. And in the absence of Spam they settled for corned beef, which also had a corpsy flavor.
In the article, the writer ponders whether or not to try and contact Paul Theroux to find out if it was meant as a joke (the book overall had a light and humorous approach), but decided not to. In the end though, Paul reached out to him with the following:
Cecil, my man!
You were right the first time. Yes, it is a joke. In spite of my solemn declaration in The Happy Isles of Oceania, the voracious Spam consumption in the Pacific is not conclusive evidence of a cannibal past.
And I enjoyed seeing my laborious joke cleverly adumbrated in yet another of your witty, wide-ranging, and inexhaustibly erudite columns.
But also, speaking as a vegetarian, all meat-eating looks to me like the first step down the road to anthropophagy.
With good wishes, Paul Theroux
Islanders developed an attachment to SPAM because meat is fairly rare in island nations and canned SPAM was shelf stable and readily available. It was a great alternative for meat consumption outside of extremely important events or holidays where it would make sense to slaughter livestock and have meat. When you don't have the land for large numbers of grazing livestock like sheep or cattle, you primarily hunt fish and keep few if any smaller livestock that don't need as much room or resources (pigs, goats). With small numbers available, you'd only slaughter rarely and generally for celebration.