I think the whole "true threats" concept is backwards. It is false threats that should be prohibited.

Lets say there is some guy at the park who really loves a particular dandelion. If you step on this dandelion this guy will stab you. I would much rather the guy announce this ahead of time, rather then just silently knife me in the kidney. If I know about the threat I can take steps to avoid it.

True threats are valuable information to have.