This is an MMO, the central letter there standing for multi-player. This means we play with other people, real people. Real people we need to treat with a basic sense of courtesy. That's one of the tricky things about that little fiasco -- we know our guildie in real life. He's a real person, an affable big fella my wife has described as a teddy bear. The problem is that in the game, 'bear' isn't the four-letter word to describe him. The satchel was just his largest haul, but he's been known to nick the occasional Abyss Crystal from a heroic run.
Although it isn't right for other players to generalize our entire guild as an organization of ninjas, it doesn't make us happy that gives us a bad name, either. As hard as it is, our GM has taken up the unhappy task of issuing an RL friend an admonition that he will be gkicked if he did something like that again.
Why is it hard for some people to be considerate? Is it because people think it's "just a game"? Why would anyone ninja anything? Do players really need loot so badly they're willing to tarnish their reputation? Or is an asshat reputation exactly what they're going for? Maybe despite our friend's cuddly exterior, he's a lying, thieving, inconsiderate bastard in real life. I don't know. I just know that even in a game where all we interact with are pixels, there's always room for good manners.