Comment by AnthonyMouse

Those are not opinions, they're provably false statements or threats. Conspiracy is essentially committing a crime as a group rather than an individual, and the statements are the evidence of the crime rather than the crime in itself.

The closest the government comes to prohibiting an opinion is copyright, but even then you can restate the opinion in your own words, and when an exact quote is necessary to make your point it's fair use specifically because it would otherwise violate free speech.