Oh come on, there is an obvious operational reason for this. The Tokyo airports are basically shut down from 12:00 to 22:00 today. This is the window when most US carriers operate TPAC flights in and out of Tokyo. If they delayed a westbound flight by, say, 12 hours, everyone would miss their onward connections in Tokyo, and the return flight would have to go back empty in the middle of the night (since many people couldn't connect to it) and then everyone on that flight would also miss connections on the US end. So better to simply cancel both flights, reaccommodate people, and keep planes in the US to operate other flights the next day.

On the other hand, the Japanese carriers tend to operate TPAC flights out of Tokyo either in the morning or late at night. The eastbound morning flights today got out ahead of the typhoon. The westbound flights scheduled to depart yesterday and arrive in Tokyo this afternoon could just sit on the ground in America for an extra 12 to 24 hours and land in Tokyo late tonight or early tomorrow morning. Then they can reshuffle the fleet and operate a regular schedule tomorrow. The handful of evening eastbound flights will be delayed by a few hours but the impact on the westbound schedule tomorrow shouldn't be that bad.