the first amendment doesn't protect murderous threats for a while now. That would be why he's in federal custody. He got arrested for threatening to kill people.
“but what should we do when the highborn and wealthy take to crime? Indeed, if a poor man will spend a year in prison for stealing out of hunger, how high would the gallows need to be to hang the rich man who breaks the law out of greed?” Terry Pratchett, Snuff