So wait a second, let me get this right here.

You say you do 9-12 normal, and yet you'd rather just spend more time in a level 15 mini mission because... I still don't know why exactly.

He called you out because you were doing nothing. You honestly don't even need a dedicated healer for that. If you can't fit in damage because it's hectic, fine. That's a good reasoning. If you can't fit in dps because you don't want to do ANYTHING, then the tank can assume you never do anything anyways and wants you to fix that.