Single target in an AOE pull is ridiculously inefficient. Things die slowly and that could potentially lead to a wipe when the tank and healer run out of cooldowns. Direct hit and Crit transfer over to aoe damage as well plus mana is no more difficult to manage in AOE than in single target so the explanation you gave was unsatisfactory. If you continued to single target after we've asked you to AOE in large packs and you refuse I'd kick you too. Especially if you whined about the tank losing hate to you on the mob you were single targeting in an aoe pack.

Their only real mistake was selecting afk rather than harassment tbh.
So