I can understand that someone saying, in a moment of passion, "I'm gonna kill you!" should not be considered a crime. But someone who writes out a warning, explicitly and calmly, saying that performing a certain act will make you a target, that is a crime. It's at least an infringement of someone else's right to free speech. And yes, the authorities may be investigating. Chances are that group has been under surveillance for quite a while now.
And I apologize to the group for going off on a rant earlier. It just drives me nuts when people like this hide behind their presumed moral authority to justify killing, terrorizing, torturing and enslaving people, all in the name of their prophet, god or savior.
Anyway here's a video that seems relevant.