I would think simple physics might be at play here, similar to that of say throwing a tennis ball back and forth on a moving airplane. Throwing the ball forwards won't cause the ball to be thrown backwards. It's actualy moving at the speed of the airplane + the speed its thrown forward at. Gravity effects the ball, therefore it loses momentum, falls to the floor and comes back to you.
But uneffected by gravity, the ball would probably keep moving at the velocity it was thrown (velocity of the throwing person + velocity of the throw)
Ditto with say, photon torpedoes. Firing forwards, the torpedo is travelling at the speed of the firing ship + the speed the torpedo is travelling when it is fired.
Similarly, firing forwards, the phaser beam actually travels at whichever warp speed + the light speed of the phaser beam.