I think generally the argument is that these usually aren’t credible threats, they’re hyperbole, and the police are overstepping by arresting people for saying they’ll do something when they have no evidence that that person would actually do that. A free society doesn’t arrest someone for saying “I’m going to burn down so-and-so’s house.” It waits until you actually go to so-and-so’s house with a can of petrol, and then arrests you.