This may sound really shallow to people who haven't worked in Japanese society, but Japan really doesn't handle surprises well. Look at the slow responses to "unexpected" natural disasters. On the other hand, you tell them exactly when, where, and how an attack will be made, and they will be better prepared for it than anybody else could be.

With many news articles on the North Korean missile that flew over Japan yesterday, one obvious question is not even mentioned: why didn't Japan shoot it down? But an article from 2017, at the timing of the previous occurrence might explain it. Maybe it simply cant'.