They might be, or they might not be. They tend to keep their mouths shut during active investigations, if people found out the FBI was actively subpoenaing the site they might be inclined to stop giving them more evidence. (That is, the FBI could have developed a public-private honeypot partnership for all we know.)
And many times people aren't espousing direct, actionable threats. Talking about "international bankers" and "globalists" is protected speech, saying they control the world is protected speech, and saying something needs to be done about this is also protected speech - as long as they don't get specific (i.e. "we should fight back, let's meet up at <place> on <date/time>"), they can't do much about it. So you have people constantly agreeing about how "Zionists" are a threat to "the west" and that "something" needs to happen, egging on each others hate with various hoaxes, reactionary takes and praising Hitler, and finally someone snaps. Despite these people clearly being partially responsible for encouraging one of their members to violence, none of them will get in trouble because they never gave direct threats or incitements.
That's legal for very important reasons. People have a right to freedom of speech and the right to assembly. They don't have a right to commit murder. There's a huge leap from complaining about something to committing violence in real life. The latter is rightfully illegal and the former is rightfully legal (in the United States).