Well, he should have picked a better sign, that did not make reference to the need for possible removal of political figures by force. That i'll give em. He could have just as easily made a different good sign that had nothing to do with that, and no one could have challenged him.
As for obama being there, that's just tough shit if he don't like it. It's the law, assholes. That man is allowed to carry that gun in state. If you have a problem with it, then don't fucking come to his state, where he may be. End of story.