Palin didn't have any clue as to what "the Bush Doctrine" meant. She tries to wriggle out like a champ, but she apparent just didn't get to that chapter of her crash course in world affairs.
James Fallows does the best job explaining why this is so bad:
Sarah Palin did not know this issue, or any part of it. The view she actually expressed -- an endorsement of "preemptive" action -- was fine on its own merits. But it is not the stated doctrine of the Bush Administration, it is not the policy her running mate has endorsed, and it is not the concept under which her own son is going off to Iraq.
How could she not know this? For the same reason I don't know anything about European football/soccer standings, trades, or intrigue. I am not interested enough. And she evidently has not been interested enough even to follow the news of foreign affairs during the Bush era.
A further point. The truly toxic combination of traits GW Bush brought to decision making was:
2) Lack of curiosity
That is, he was not broadly informed to begin with (point 1). He did not seek out new information (#2); but he nonetheless prided himself on making broad, bold decisions quickly, and then sticking to them to show resoluteness.
We don't know about #2 for Palin yet -- she could be a sponge-like absorber of information. But we know about #1 and we can guess, from her demeanor about #3. Most of all we know something about the person who put her in this untenable role.