AWOL:
I thought they used gyroscopic precession, biasing the speed of the clockwise and anticlockwise motors.
That's nothing to do with precession, that's just applying a net torque by balancing the torques
from the two sets of props that counter-spin.
Back to the original question - you are asking why a particular quadcopter's control software is
programmed as it is - that is a question for the author of that software, not for the Arduino
forum.