Why do most spaceships in sci-fi have their guns on backwards?

Have you ever noticed that most spaceships in science fiction only have aft thrusters and forward-facing guns (or on the sides)? Wouldn't you want guns that turret 360, aft-facing guns , and forward-facing thrusters so you can move the ship towards and away from enemy ordinance? Spacedock looks at this common oversight in spaceship design and at one ship design that gets it right.

Image: YouTube