Lane over on WoW LJ pointed us to this article about "Playing to Win" -- the article itself is actually about Street Fighter II, but as Lane points out, the concept could very easily be connected to World of Warcraft. I remember Turpster talking about this on our podcast as well a few weeks ago: playing for fun and playing to win are two completely different things, and the difference between them can put a lot of players at odds.
Players who "play to win" wonder why some folks in game try to play by "fictitious rules" -- why not do some arena matching, or why not jump on an opponent on a PvP server when they're almost dead from fighting a mob? If there's a mod that will let you "cheat" on a boss, or a macro that will let you target opponents in the arena, why not use it? But just as the person writing this article about scrubs sees those as "fake rules" (because the game doesn't actually keep you from doing those things seen as "cheap"), many players do see things like honor and discipline in the game.
In the end, it's each to his own -- we all play this game for different reasons, and not everyone does play to win: some play for fun, some play to relax, some play just because they want to follow those "fake rules" (and any others they come up with -- pacifist players, Arena masters, or naked trolls). Players play by their own rules all the time, but that's no reason at all to call them "scrubs."