Scapegoating North Korea obviously benefits plenty of sides, mostly the US government which can turn the attention of some easily fooled citizens away from the internal problems of the country and justify spending on defense contracts. South Korea and Japan are the only countries that really have reasons to be scared of the North since their missiles(and maybe nukes) can potentially get to all their major cities. The overreacting by the US to all things the North does plays to South Koreas and Japans interests, it's understandable.