Goodgulf:
The reason? It teaches students that life is not fair.
My son went to a fee paying school and I came to the conclusion that they played rugby instead of the more popular association football for exactly this reason.
The rules are so complex, and the action is so confused that at any moment the ref can penalise just about anyone involved in the action. I would say that every time I picked him up from a rugby match he had some complaint about the refereeing being unfair regardless of the result of the match (and he stopped playing club rugby because of exactly this).