It's been several generations since WWII. Japan is one of the US' closest allies. If they wanted to transform their self-defense force into a full-blown military and take over responsibility for their own defense, I'm sure they could do so. So far, no one has generated the political will to do that. Your buddy Kim isn't helping things by sending missiles over Japan.

This part is ironic. The tankies will often argue that Japan shouldn't be expanding their military (and anything their military does they think is wrong), but also that the US shouldn't be involved. You don't get both. You can't just say a nation should have no way to defend itself, especially when you are defending Russia and China.