Pumped Up Kicks is explicit satire, not an actual call for school shootings.
Let me ask this: what makes more sense, a random, specifically Japanese group of people, enjoys northern European Neo-Nazi music for the tunes? Or, perhaps, the fact that Japan has legitimate fascist movements, and historical ties to fascism without heavily denouncing them like Germany, means fascists are likely to search out more fascist music?
I understand your point, but it’s incredibly hard to just randomly stumble upon fascist music and enjoy it for the vibes. This is a specific level of dedication in a country with higher than normal levels of support for fascism.