I would guess the temptations was too great and that he thought the results would be buried by the Tour directors so its reputation wouldn't be further sullied. It IS insane (although not in a legal sense) to do this but he saw his Tour victory go up in smoke the day before and this was the ONLY way he could get it back. And he got caught. He probably thought if there was even a 1 in a 100 chance he could slip it by it was worth it. In hindsight, he will regret it as his career is almost certainly over and his reputation forever tarnished. A real human tragedy.