having a moment here in gnome
to everyone pointing out that this is for touchpads;
a: it's awful on that too
b: note the mouse in the example given
It's because of joysticks and typical flight controls. Pushing forward goes down and pulling backwards is "pulling up".
Joysticks rules for a long time before the mouse came out. Home computers came standard with joystick ports.
Keyboard controls followed this convention and when mouse controls came into FPS games this was the first instinct... Moving the mouse "forward" looks down.