There's a very amusing opinion piece about this by Gail Collins in the NY Times.

Here's one quote from it:

Basically, the point was that Palin is quitting as governor because she’s not a quitter. Or a deceased salmon.

Here is another:

According to the about-to-be-ex governor, fighting all this negativity has cost the state “thousands of hours of your time” and $2 million “to respond to ‘opposition research.’ &rdqu o; But now this is all water under the bridge. Every single unfair charge has been dismissed. (“We’ve won!”) And now that the battle is over and the time/money has been wasted, Palin is going to leave her job in the name of “efficiencies and effectiveness.”

You have to give it Sarah, she doesn't waste time worrying about logic.