Dying in World of Warcraft

So for those of you who don't play World of Warcraft, when you die, you get sent to a spirit healer and you can come back to life. This makes many people question the re-spawning of other things. Why is Cairne dead forever? Why isn't Garrosh dead when I smacked him last week when we were doing a bear run? Why don't the dead Dwarves in Twilight Highlands get back up and keep on fighting the good fight?

I think I have an answer to all this.