> After a generation, those immigrants will have children who are Japanese. Their native home will be Japan.

Since Japanese culture/society will not accept anybody into their "inner circle" those immigrants will build parallel societies. A diaspora usually heavily connected to their original homeland and not their host. If those get really big, they will have influence in politics which again will change the face and culture of Japan forever.

This has nothing to do with racial purity. It has to do with what is in the best interest of the Japanese people.