This threw me at first too, but quickly became less weird feeling and after 5 minutes or so it was second nature. I think its a holdover from when the mouse was visible while clicking and dragging. So dragging down and right to launch left made sense: like launching a bird in Angry Birds. But with the mouse hidden, that part of aiming is abstracted and people are conditioned to move in regards to the indicator so we expect moving the mouse left to move the aim left.