The reasoning, I think, is that the penalty for hitting into a hazard is not being able to judge the nature and consistency of the hazard, so a little guess work has to go into any sand-shot.
Really though, it's just one of golf's dumb rules. Did you know that if you hit the ball, and the ball hits a tree in front of you and ricochets back and hits you, it's a 2-stroke penalty? My dad enforced that on me once.