The end result is that 99.999999% of players affected by these jerks do nothing. And this lack of response to bad behavior only emboldens the jerks more.
It sucks, but this behavior will go on so long as the person behind the keyboard remains anonymous.
Well, I'm one of those quiet types that won't complain or even whisper to them how I felt about their behavior but I do believe what goes around comes around.
From my own non scientific observations there are two types of people that ninja loot.
The first group you won't get much satisfaction from, they are most likely mmo veterans and probably way more skilled than you or I. They could easily clear all the content but they are 'speed rushing' through it trying to grind something in the least amount of time. It doesn't make it right but they are out there. The best you can hope for is you're on a pvp server, you see them get ganked and you don't help them. That's what I call just deserts. It's even better if they are wearing their legacy name. So if I run into any of their alts, same rules will apply.
The other type of ninja are the terrible players. You've seen them; thier companion dies in seconds, they die a few seconds later, they can't complete anything and end up skipping loads of content as a result. Their only way of completing objectives is to ninja them from others. These people I almost feel bad for. If they would just ask in general chat or whisper someone, they'll probably get the help and advise they need.