He was in the last group on the course and had sole possession of the lead, so obviously everyone is watching his every move. What he did was blatantly obvious while watching live, so anyone watching with the rules in mind was going to catch it. The replays were run over and over by the officials so that they could show it to him and put to rest any protest he might have.
edit: And with what Linksfield said about signing the card, that is the one rule that needs to go. In these events anyway, that is the biggest dinosaur there is. It's about as useful as the official status that a goal judge used to have in hockey. Luckily the NHL removed their impact on the game years ago.