This kind of thing (people looting stuff they don't really need) doesn't happen as much when you play with a more localized community. It's the same problem with people that high their identity on the internet and troll people relentlessly. Do those same trolls act this way when everyone knows who they are? Probably not, in fact, they are probably relatively tolerable human beings. The whole thing is related to the fact you'll never see the people in your Dun Scaith again most likely. It brings out the worst in people. As long as people can just randomly queue up with strangers whenever they feel like, they aren't going to feel compelled to behave beyond what is required of the game system itself sadly.

Side note though, while you may assume the worst intentions of this person that "ninja-looted" they may also assume your intentions were nefarious as well even if you were nice about it. People are so untrusting these days in games. I really miss the "golden age" of mmos where people were a bit more worried about how they were perceived.