Because they have a warped sense of what is right. Why go 38 minutes where a moving screen is a foul or a reachin on a pass is a foul, only to swallow the whistle for the last couple minutes? It makes no sense.
Ive heard it explained that they want the players to win/lose the game and they dont want to affect the outcome. Well thats great and all, but if the players are breaking the rules you are supposed to enforce, then frigging call it! That isnt affecting the outcome, the player who fouled is affecting the outcome.