From what I can gather from reading message boards and my own experience with the game, I think a lot of the problem is that the griefers are exploiting the casual aiming setting, so they're not killing people because they're better at the game, but because they can press 2 buttons simultaneously and have the will to grief.
The casual aiming setting gives you automatic lock-on, automatic target tracking, and turns the aiming dot red when you have a kill shot, so all you have to do is hold down the left trigger and the game will automatically lock onto a person and track them, even through cover, even when you can't physically see them and haven't spotted them.
To be a casual aim griefer all you have to do is stand on a high point or hide by couching in some bushes (it doesn't matter if all you can see is shrubbery), repeatedly tap the left trigger until the reticule turn red, then hold the left trigger and fire... you don't even have to see or be aware of your victims, all you have to do is fire when aim dot turn red and you've got a kill.
If free roam enforced manual aiming then it wouldn't be so much of a problem because you'd have a chance to defend yourself, seeing as the first shot wouldn't necessarily kill you could get into cover, retaliate or escape... but when people are using causal, laser sighted, fire and forget aiming, usually the first indication you get that someone with hostile intentions is in the vicinity is when their magic bullet makes you take a dirt nap.