It's a ploy by the enemy. Simply do not reply to it. If it's a clear threat of violence to a specific person, place, and time, click the arrow to Report the post.
Like you say, there are legal limits to the privacy here, in rare cases. An anon who replies to a post that is a part of the enemy's false flag, could be labeled a potential conspirator. This is partly why they are doing it - to scare anons from participating here.