The Supreme Court has long distinguished content-based speech restrictions, which are almost never upheld (when they’re imposed by the government as sovereign rather as proprietor, and deal with speech outside the established First Amendment exceptions), from content-neutral speech restrictions, which will often be upheld if they leave open ample alternative channels for communication. And the Court has at times made clear that a restriction is content-based even if it turns only on the subject matter of speech or general content of speech (such as whether speech serves “philatelic, numismatic, educational, historical, or newsworthy purposes”), rather than with the specific ideas expressed by the speech.
Still, some lower court decisions — and perhaps at times the Court itself — have treated facially content-based restrictions as content-neutral if they deal with broad enough classes of speech. Holder v. Humanitarian Law Project seems to reject this:
United States v. O’Brien does not provide the applicable standard for reviewing a content-based regulation of speech, see R.A.V. v. St. Paul, 505 U. S. 377, 385–386 (1992); Texas v. Johnson, 491 U. S. 397, 403, 406–407 (1989), and §2339B regulates speech on the basis of its content. Plaintiffs want to speak to the PKK and the LTTE, and whether they may do so under §2339B depends on what they say. If plaintiffs’ speech to those groups imparts a “specific skill” or communicates advice derived from “specialized knowledge” — for example, training on the use of international law or advice on petitioning the United Nations — then it is barred. On the other hand, plaintiffs’ speech is not barred if it imparts only general or unspecialized knowledge.
This, I think, is in some tension with the Court’s conclusion in Hill v. Colorado (2000) that a limit on “protest, education, or counseling” outside health care facilities was content-neutral. I think the two cases are theoretically reconcilable, but I’m not sure whether such a reconciliation is likely to be entirely satisfactory or persuasive. But in any event, it seems to me that Humanitarian Law Project‘s discussion of content discrimination further points towards a broad definition of content discrimination.
For most restrictions, there’s little controversy about whether they are content-based or content-neutral. If the restriction treats speech differently based on the viewpoint or subject matter of the speech, on the words the speech contains, or on the facts it conveys, the restriction is based on the content (and the communicative impact) of speech. Classic examples: Bans on advocacy of illegal conduct (Brandenburg v. Ohio), on sexually explicit speech (Miller v. California), on all residential picketing except labor picketing (Carey v. Brown), on profanity (Cohen v. California), or on the publication of the names of rape victims (Florida Star v. B.J.F.). Some of these restrictions may be viewpoint-neutral, but they’re all content-based.
For more about the distinction between content-based and content-neutral restrictions, and the mysterious, mystifying, and perhaps even mystical “secondary effects” doctrine, see this post.