"publication of sailing dates of transports or the number or location of troops". This exception was a key point in another landmark case four decades later: New York Times Co. v. United States (1971), in which the administration of President Richard Nixon sought to ban the publication of the Pentagon Papers, classified government documents about the Vietnam War secretly copied by analyst Daniel Ellsberg. The Court found that the Nixon administration had not met the heavy burden of proof required for prior restraint. Justice Brennan, drawing on Near in a concurrent opinion, wrote that "only governmental allegation and proof that publication must inevitably, directly, and immediately cause the occurrence of an evil kindred to imperiling the safety of a transport already at sea can support even the issuance of an interim restraining order." Justices Black and Douglas went still further, writing that prior restraints were never justified. The courts have rarely treated content-based regulation of journalism with any sympathy. In Miami Herald Publishing Co. v. Tornillo (1974), the Court unanimously struck down a state law requiring newspapers criticizing political candidates to publish their responses. The state claimed the law had been passed to ensure journalistic responsibility. The Supreme Court found that freedom, but not responsibility, is mandated by the First Amendment and so it ruled that the government may not force newspapers to publish that which they do not desire to publish. Content-based regulation of television and radio, however, have been sustained by the Supreme Court in various cases. Since there is a limited number of frequencies for non-cable television and radio stations, the government licenses them to various companies. However, the Supreme Court has ruled that the problem of scarcity does not allow the raising of a First Amendment issue. The government may restrain broadcasters, but only on a content-neutral basis. In Federal Communications Commission v. Pacifica Foundation, the Supreme Court upheld the Federal Communications Commission's authority to restrict the use of "indecent" material in broadcasting. State governments retain the right to tax newspapers, just as they may tax other commercial products. Generally, however, taxes that focus exclusively on newspapers have been found unconstitutional. In Grosjean v. American Press Co. (1936), the Court invalidated a state tax on newspaper advertising revenues, holding that the role of the press in creating "informed public opinion" was vital. Similarly, some taxes that give preferential treatment to the press have been struck down.