Quitting doesn't matter... things come up, computers freeze, ppl don't want to finish the round for whatever reason...
It's not cheating and it doesn't affect your rankings. Sure, it can be a bit annoying but so what? As claremoreblue said.. get over it.
If the only reason you play multi-player is to be better than someone else, and you get angry when they leave, you really need to look closely at your own game. What does it say about you that you are better than me in an on-line golf game?
I can't speak for all, or even most... or even a select few.. but I can tell you that when I play multiplayer I am still playing the course and trying to get to MY personal best. And to repeat from another note... "If my best is better than yours, or vice-versa, it only matters that we had fun and maybe shared a laugh or two along the way. That is what is referred to as a 'good game'. After all, this is only an on-line game. If you have to judge your worth by your achievements here, you have much bigger problems than worrying about who quit while you were playing."