Hi Ausjulius,
Just guessing, but there are a couple of things going on here culturally.
Remember that the Solomons are one of the places where "cargo cults" showed up. I'm not saying that the islanders are stupid (they're not), but a couple of generations ago they went through big time culture shock, and I'll bet that, to this day, they value outside "Western" goods more than they value "bush" goods, except when tourists buy bush goods as art and artifacts (not that we do this on this website
)
This is helped by traders selling things like rubber bands and speargun slings pretty cheaply.
My guess is that either they saw the "Pana" (or whatever it's called) because some Philippino fisherman had one. Or a sailor saw one in the Philippines and brought one home (or took the idea home). Or they just got the idea from looking at spearguns and fishing spears. There is enough boat traffic in that region that I'm not surprised that the idea transferred. It's really an urban weapon, anyway, since the metal for the dart and the rubber for the band have to be imported.
Anyway, the pana is "western," it's cheap, it works, and actually, it is a lot easier to conceal than a 4' longbow, which is what they'd be using if they went the traditional route.
F