He left a job for a better one. Who wouldn't? It happens every day, esp. in high profile sports.
As for Pola, he was offered the position when LK first came to SC, so this shouldn't be a shock that he changed his mind, esp. in lieu of the fact that if the NFL locks out next year, that assistant coaches can and will be left hung out to dry. They have no union to protect their interests and the owners and players couldn't care less either. Bitch all you want, but LK and Pola are guilty of looking out for their own best long-term interests and that's it.