Police Department v. Mosley (1972)

But, above all else, the First Amendment means that government has no power to restrict expression because of its message, its ideas, its subject matter, or its content. To permit the continued building of our politics and culture, and to assure self-fulfillment for each individual, our people are guaranteed the right to express any thought, free from government censorship. The essence of this forbidden censorship is content control. Any restriction on expressive activity because of its content would completely undercut the "profound national commitment to the principle that debate on public issues should be uninhibited, robust, and wide-open." New York Times Co. v. Sullivan.

Necessarily, then, under the Equal Protection Clause, not to mention the First Amendment itself, government may not grant the use of a forum to people whose views it finds acceptable, but deny use to those wishing to express less favored or more controversial views. And it may not select which issues are worth discussing or debating in public facilities. There is an "equality of status in the field of ideas," and government must afford all points of view an equal opportunity to be heard. Once a forum is opened up to assembly or speaking by some groups, government may not prohibit others from assembling or speaking on the basis of what they intend to say. Selective exclusions from a public forum may not be based on content alone, and may not be justified by reference to content alone.

Simon & Schuster, Inc. v. Members of New York Crime Victims Board (1991)

[T]he Government's ability to impose content-based burdens on speech raises the specter that the Government may effectively drive certain ideas or viewpoints from the marketplace. The First Amendment presumptively places this sort of discrimination beyond the power of the Government. The Son of Sam law is such a content-based statute. It singles out income derived from expressive activity for a burden the State places on no other income, and it is directed only at works with a specified content. Whether the First Amendment "speaker" is considered to be Henry Hill, whose income the statute places in escrow because of the story he has told, or Simon & Schuster, which can publish books about crime with the assistance of only those criminals willing to forgo remuneration for at least five years, the statute plainly imposes a financial disincentive only on speech of a particular content.