Because that path is likely to leave a shot depending on which side of the ball is struck. Rolling directly onto the ball will not leave a shot (other than a double). At pool it is rare that anyone plays to leave the cue ball on top of the object ball for a safe since usually it is very easy to play a thin hit and take the cue ball far away. For the shot Fisher played, there is a chance it could leave a difficult return safety. I think I'd kick off the side cushion with a speed that gives a chance for separation.