Many people in our great land might have difficulty defining the "Bush Doctrine" exactly. But not to recognize the name, as obviously was the case for Palin, indicates not a failure of last-minute cramming but a lack of attention to any foreign-policy discussion whatsoever in the last seven years.
That's exactly it. She never heard the phrase until the moment when Gibson mentioned it. If you're in politics, there are certain buzz-phrases you're expected to know. It's like claiming to be an internet user and not knowing "LOL."