I don’t know why the Japanese government would not expect this and plan for it (or perhaps they did behind the scenes and this is more theater). They (primarily LDP and other right-leaning parties) talk about intervening against mainland china, they talk about wanting to revise the constitution about self-defense, etc. and then act surprised at the very predictable reaction.

(I am a non-Japanese person in Japan for over a decade. Also not a huge fan of most things mainland china does, for the record).