This a default feature in most video players to ensure driver's and occupants' safety by avoiding any possibility of distraction while the car is being driven.
The video is enabled only when the car is stopped and the e-brake (handbrake) is pulled up. Video is enabled with the help of a wire spliced in to e-brake switch where it gets the ground when the e-brake is engaged. Electricians permanently ground this wire to keep the video always on, but that is not recommended.