Yeah, political rhetoric has consequences. But violence by the mentally ill isn't one, or at least it's such a rare, unpredictable, indirect, and unproven consequence that we shouldn't worry about it. People should not have to tailor their speech based on what an insane listener might do.
The harm Palin and her allies do is through the normal political process. If we want to talk about the language of politicians leading to killing, we should be talking about Iraq and Afghanistan.