Also, I'd give better than even odds that GP's "right" answer is a pet solution of theirs that other people of similar background and skill level would contest.

I don't disagree with the practice of giving simple questions with solution spaces well understood by the interviewer. I use it myself. However, I force myself to accept any answer that correctly identifies tradeoffs as correct and I have to intentionally suppress my inclination towards a specific set of weights on those tradeoffs in order to be fair. This doesn't seem to be nearly as common as it ought to be.