Failsafe ARDrone

If no joystick is used in Failsafe mode, the drone will slowly descend with a speed of 0.5m/s instead of setting the thrust to zero. This will prevent the drone from dropping out of the sky without the possibility to take over with the joystick. 
An example flight plan for the optitrack system. The flight plan specifies the flight area indicating where and at what height the waypoints can be positioned. 
Finally the gains for indoor flying with the ARDrone in the arena have been tuned.
why is the failsafe value not zero ?