The rule is needed to protect the players in the air. In my soph. year of HS in Illinois, I had that happen to me. At the time, I was starting for a top 10 ranked team. The injury put me on the bench for the rest of the season.
When you leave the floor, your instincts prepare you for the landing. But when your feet and legs are taken out, you are instantaneously put into a blender with no idea where you are landing or on what part of the body. Serious injuries are the result way too often. Blocking out is one thing, undercutting is a completely different animal.