Keep in mind that SEA-NRT is not just “Star Aliiance” as UAL and ANA have a Joint Venture (JV) that covers the North Pacific.

Within the JV revenues, expenses, and profits are split regardless of the color of the airplane. I hate to say it, but it probably makes a strong business sense to have ANA operate the city pair in the absence of a UA 777 base in SEA. (The idiotic UA abandonment of SEA in the first place is a whole different can of worms.)