A queue that is inactive, due to the ship cap being reached etc, still contributes to your production capacity. It will immediately start cranking out ships as soon as some of your existing ships are destroyed. If that constructor were to automatically go into low power mode when the queue became inactive, there is no guarantee that is could come back online when it's required to continue producing that ship type.
If the queue is completely empty, then the player could manually put the constructor into low power mode, I'm not sure that automatically going into low-power mode when the queue is inactive is in the player's best interests.
