The thing is that US law does not protect such things. The First Amendment protects our right to freedom of religion, speech, press, assembly, and our right to confront our government with grievances. It specifically prohibits Congress from making laws against these things. However, for over 60 years now, the precedent has been established that the First Amendment does not protect words actually meant to incite violence. It is called the fighting words doctrine (Chaplinsky v. New Hampshire) and it means that I can prosecute you if I can prove that you called my mother a whore (or whatever).
So those billboards are riding the raggedy edge (if I were a judge and I were pressed for a decision on those billboards, I would say that they are borderline since they are not directed but it's splitting a very, very fine hair) but things that the church might say would probably go over the line. "He deserved it" or "He's burning in Hell right now" would be considered by me to be fighting words - and I'm not sure many judges would disagree.