Fiduciary duty applies to all agents acting on the behalf of principals, all the time. If an agent were to honestly believe that it was in the best interests of shareholders (financially) to keep Yahoo! jobs, it would be fine for him to vote that way. If an agent were to believe that layoffs were best for shareholder value, but voted otherwise in order to fulfill another objective, that's absolutely a breach of fiduciary duty, which is a legal duty.

Incorrect. It is actually against the law.