In this case it is Koreans in Japan who refuse to assimilate. And like Mexican revanchists, they refuse to live in what they call their wonderful homeland. The Norks apparently enjoy the advantages of a modern industrial democracy much more than homeland. And obviously they refuse to go home. And the Japanese are getting sick of it.
More power to you Nippon!
Now it is true that the Japanese don't like foreigners living among them. They will treat visitors with graciousness and politeness, but don't really like them and quite frequently don't want to associate with them in any way. Koreans there are usually not allowed to naturalize, even those born there and desire to assimilate. That can make life difficult, but rampant Korean nationalism justifies that attitude by the Japanese. Koreans, Norks or normals, are very nationalistic and resent their period under Japanese colonial rule, although they were once quite proud that they were the Number 1 tributary to the Chinese Empire.
But the lesson is that unassimilated immigrants and their descendants, like for example, Major Nidal Hassan and Faisal Shahzad, are a danger to society. The Japanese have partially solved the problem by limiting immigration. But they should end the problem by removing such Koreans who have nothing but dripping contempt for their hosts, much like Muslims and Mexicans in the U.S.