[jfengel]

It's the latter two. It's generally out of the jurisdiction of local police departments, and the FBI doesn't have anywhere near the manpower to take every threat seriously.

Personally, I'd really like to see them put at least a little effort into it, to send the message that it is in fact illegal and you take at least some risk. As it is, people often make no real effort to disguise themselves, but get away with it because no authority cares enough as long as they don't commit actual violence. So they can shut down speech with impunity.