This is a thin line in my opinion. Freedom of expression is important, but it depends. They specifically didn't say go out and commit violent acts against conservatives so nothing illegal is being done technically. However it is being displayed that if you do you will be celebrated so they are in essence rewarding acts of violence with praise.
Perhaps we could better analyze freedom of speech with The Asch conformity experiments. These experiments were a famous series of studies conducted by psychologist Solomon Asch in the 1950s to show how group pressure can force an individual to conform to an obviously incorrect answer. If this is to be considered you could in essence make the case any group advocating or celebrating violence against another group is influencing people through behavior and not speech. You'd then be removing individual responsibility and placing it on the group. You'd have to make the case that behavior is speech and that if the behavior promotes or rewards violence through praise then that particular behavior is not protected by law.
It's an interesting topic. I'd love to hear the opinion of you and others on this.