Incorrect. The crime is "making terroristic threats." It's also False Information and Hoaxes, and could put you away for 5 years. This type of speech is NOT protected by the First Amendment.
Are you kidding?! It's illegal to do so. Punishable by up to twenty years in prison.
The way I interpret it is that you're still allowed to say it under the guise of free speech, however your actions are punishable as they are in direct violation of another specific law. I'm no expert though.
It's not that you're not allowed to yell out about bombs, think about bombs, or even believe that there is a bomb, it's that in the particular scenario doing so involves breaking another specific law and in doing so you can be punished as such.
I guess by making such laws you are infringing on peoples right to "Free speech" as such, but it's more a case of them being punished for their actions which are deemed to be dangerous (threatening the lives of other people) rather than simply for their words.
If WBC were doing what they do purely out of malicious intent, I believe they would have been punished for their actions already, however as they are a "religious" organisation and are exercising their beliefs, they realistically have every right to do so.
The trick is in telling whether or not somebody does actually believe what they say they believe, or if they are just saying so because it allows them to do what they do. Unfortunately we can't tell what's inside another persons head, so there is no way of knowing if by stopping someone doing what they do you are in fact persecuting them over their beliefs.