Short summary: it appears that courts do not treat corporate shareholders as the ultimate authority of a corporation, deferring to the authority of the Directors. The OP wonders why, then, the Directors behave as if they do answer to the shareholders.

This is interesting, I think, because the question of fiduciary responsibility comes up here on HN from time to time.

It seems to me that the author is starting out with the assumption that corporations are evil, or at best amoral. If one drops this assumption, the explanation is easy: even without courts forcing them to do so, corporate Directors tend to want to do what's best for their shareholders.