Aiming down sights was always a console thing. It's extremely hard to make small aiming adjustments on a joystick, so game devs added a button to move the camera in and reduce sensitivity. CoD popularized it, and every FPS shooter wanted to ride CoD's coattails so it's pretty ubiquitous now. For a game like CS it would homogenize the guns a bit, since there's a clear delineation between guns that can be scoped and those that cannot.