I ran into this today.
Context:
When i achievement hunt i do it in low pop games, because less face it if you haven't got more than 2 people you haven't really got a tactic worth talking about, so there's nothing to ruin.
Anyway pretty simple cheevo just kill ships as a gunner, So i drop into a low pop game take the flak of a gat/flak pyra. Now i knew there was already a gunner on board, but we all know you cant trust the AI to fire the gat so logically the gunner was on that, so no real issue, you'd have to be slightly anal to take offense to someone who's going to help time the flak shots even if they are a second gunner.
This guy Rokt (or something) *EXPLODED* and proceeded to tell me from his moral high ground how rude i had been by being a second gunner and question why i would do such a thing.
Partly to fuel his outrage i was honest and told him i was hunting achievements, but i offhandedly threw in he should be grateful that he now had timing. Despite running the build he didn't seem to understand the concept and railed on a bit more, and on lobby (which was a very short time later) he left.
I think its this sort of thing that the OP was getting at, When the reactions are divorced from reality - I mean i've never even been to COD how can i go back