ode
Because there's a difference when a defender is making an honest play on the ball, even if it is considered a hard foul, versus sticking a foot out once a player has blown by him and intentionally tripping him/not making a play on the ball.
And what you deem b.s. is actually covered in both the ACC conference's player code of conduct, not to mention the NCAA's. Both authorities reserve the right to review repeated unsportsmanlike incidents, particularly those that have potential to injure another player, and if they sense that appropriate disciplinary measures aren't being met - they reserve the right to suspend him from play permanently.
One other thing... a player driving to the basket reasonably expects some level of contact if a defender has committed to make a play on the ball, and at least has an opportunity to brace his landing if the contact forces him off balance. A player who has blown by a defender isn't expecting an opponent behind him to trip him. I've seen just as many players land awkwardly in those (unintentional) feet tangled up instances, with their heads hitting the floor, as those going to the rim and being aggressively challenged.
The bottom line is it's just bush-league play and it should never be tolerated, despite the associated risks of injury involved.