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.
You may be wrong when you say only the Japanese and the South Koreans have to worry about North Korea. Recently, North Korea has succeeded in assembling ballistic missiles, which are capable of targeting installations far beyond Japan. Even American territories such as Hawaii and Guam may be within their range.