[aspenmayer]

The actions of the KKK are not legal if they prevent free exercise of civil rights.[1] The Supreme Court has weighed in on this issue and found the actions of the KKK to be in violation of the law if they impede protected activity, such as protesting or voter registration.

[1] https://en.wikipedia.org/wiki/Third_Enforcement_Act

Edit: It should be noted that this statue only applies to actions of representatives of the government[2]. It does not necessarily apply to the general public; the context of this comment chain is vague but implies a context that at least could include the public. I don’t mean to misrepresent the law. I am not a lawyer.

[2] https://en.wikipedia.org/wiki/State_actor

[darawk]

That's true, interesting and important. But I don't think it addresses the intent of my point, which was that there are plenty of odious organizations that are abiding by the letter of the law in public, but may well have members or private discussions that are likely to tend into serious criminality or terrorism. The KKK is one example, as are right-wing militias, etc. I think it's important that the government be able to conduct some basic level of surveillance on their activities, even without any direct evidence of a crime, or intent to commit a crime.