Actually, the police do go after anyone making threats to elected officials. I don't have a link handy, but there was a thing a few years ago where someone on LJ wished Bush would die in a specific manner and she got raided by the Secret Service. Sarah Palin gets away with it because she's not explicitly calling for anyone's deaths. There's a fine line and I'm pretty sure Palin knows exactly where it is and just barely stays clear of it.