http://www.chess.com/blog/webmaster/live-chess-gets-some-love
They went with simple: Single minor piece or two knights. While technically some mates can be arranged (such as in your example), draws granted in these situations can be considered more like those from "Insufficient Losing Chances" - a rule where you can be granted a draw by the arbiter/TD if your opponent is just trying to win by running you out of time.
@omnipaul Thanks very much for the response and link. Makes sense. I was just reading an endgame book with an example like the one I gave above, and that's what prompted me to ask.
@harryz Thanks for the thread. It's pretty long and full of opinions. At least now I think I know the rules that are implemented here on chess.com, which seems like the fairest way to handle it given that there are no arbiters and given that it needs to be simple and consistent. I'm guessing all chess servers handle the situation like this.
My question was somewhat covered in other posts, but not quite.
Suppose your opponent runs out of time, but you have only a king and knight left. Chess.com will not award you a win on time because you have insufficient mating material. Instead a draw is given. But I want to know the exact rules about what constitutes insufficient mating material. Is this written down somewhere? I wonder about situations like this:
Here White can win with 1.Nb5+ Ka8 2.Kc8 a7 3.Nc7#
But suppose Black runs out of time. Will the server recognize that White can win and grant the full point? Or is it programmed to see only the king and knight and say "insufficient mating material"? If the latter, than once Black saw the mate coming, Black would prefer to let his or her time run out before giving White an opportunity to mate.