Whoever rolls need on a green knows very well that by doing so he excludes the majority of other players from having a chance of getting the item, because they either roll greed naturally or can't even need due to armor restrictions - so instead of rolling vs. 24 players it'll probably be vs. 0 to 5. At this point the need roll becomes an act where you increase your chances of winning in a significant way by actively chosing to go against courtesy.
No wonder people get mad at you - it's even worse if the item is of such a low value, because who could tell what you would do if it was actually something of value? Of course such thoughts seem silly, considering that you might not see the people you made fools of ever again; but I know at least one case in which someone was rejected from guild invitation because he demonstrated a massive case of misbehaviour a few weeks earlier in LFR.
It is beyond me how people can deliberately choose to misbehave like that over items of such low value.