Bull crap. I've threatened to kill people before. So have you.
It is our right to express our disgust for people by saying that we'd like to kill them or like to see them die.
That right is secured by the first amendment. It is only when we act on that threat that we should be subject to prosecution.
I am willing to bet that there has never been an assasination of a president by someone who has publicly threatened to kill them. Assassins don't threaten to kill presidents. They just do it.