I love how everyone's attitude seems to be "Oh well, that's what happens." Either that or they debate the term "ninja" as though somehow that were relevant to the real problem.
It happens because you allow it to happen.
If someone behaves this way in a group, tell them to stop. If it continues, then vote kick them and put them on ignore. If you just passively let it continue, then you're part of the problem - not only for yourself (and maybe it isn't a problem for yourself because you don't need the cash or the loot) but for other players who will encounter these d-bags.
People want a better community? Then start making one.
If it WERE that simple, we wouldn't have this problem. It's not like someone gets a special flag or marker telling you that they are jerk and plan on stealing most everything from you. On top of that, most of the people who play MMOs have to start somewhere and the "rules" of gaming are not known to them. We can inform and warn and ban and block etc all we want but that won't stop the real troublemakers. If that were possible, I'd never read something awful or stupid in general chat. People can only DO so much for "their" community. You can't force other people to adhere to civility and manners.