Not just rugby, but cricket too. Cricket is 100% based on respect and playing in the spirit of the game. There was an incident once where a player was given out LBW when it was very very obvious that the umpire had made the wrong decision. The equivalent in football is maybe the ball hitting a player's stomach and the ref giving handball.
So this player, when he was walking off, he pointed to his bat to show that the ball hit his bat. That is all. What happened? He got disciplined and got fined 20% of his match fee and had to apologise. Cricket is all about respect for the officials. Players can show disappointment in themselves for getting out, but if they show any sort of disappointment or resentment towards an official - BOOM! Disciplinary time.