BOSTON (CN) — Once a government opens up a forum to the public, it might have to allow viewpoints it doesn’t agree with, the Supreme Court ruled Monday in a decision that scolds Boston for being selective about which flags citizens can fly outside city hall.

Justice Stephen Breyer wrote the lead opinion, which is joined by five colleagues. No member of the court dissented, but Justices Brett Kavanaugh, Samuel Alito and Neil Gorsuch all wrote concurring opinions.

The case involves three 83-foot-tall flagpoles outside Boston’s City Hall. One of the poles usually flies the city’s flag but, on hundreds of occasions, the city has allowed private individuals to temporarily fly other flags in conjunction with an event — typically flags of other nationalities but occasionally ones that celebrated subjects such as Juneteenth or gay pride. 

When a man named Howard Shurtleff wanted to fly a so-called Christian flag featuring a red Latin cross, however, the city said no. 

The city was wrong, the Supreme Court ruled. By allowing so many other people to fly flags, the city had turned its flagpole into a public forum and now it is barred from limiting speech that it does support or discriminating on the basis of religion.  

“All told, while the historical practice of flag flying at government buildings favors Boston,” Breyer wrote, “the city’s lack of meaningful involvement in the selection of flags or the crafting of their messages leads us to classify the flag raisings as private, not government, speech — though nothing prevents Boston from changing its policies going forward.”

Earlier in the opinion, Breyer described the court’s study of “the extent to which Boston actively controlled these flag raisings and shaped the messages the flags sent.”

“The answer, it seems, is not at all,” he said. “And that is the most salient feature of this case.” 

Boston acknowledged that it “hadn’t spent a lot of time really thinking about” its flag-raising practices, Breyer dryly noted. 

Breyer pointed in contrast to the city of San Jose, California, which provides in writing that its “flagpoles are not intended to serve as a forum for free expression by the public” and lists approved flags that may be flown “as an expression of the city’s official sentiments.”  

How governments use their channels to allow some people to speak but not others has been a major bone of contention in recent years — one that has given rise to frequent charges of favoritism and censorship. 

The high court has given mixed signals on the issue since 2009 when it held that a city didn’t have to accept a monument in a public park proposed by the quasi-religious group Summum despite having previously allowed other privately funded monuments.  

In 2015, the court said the state of Texas could reject a specialty license plate with a Confederate flag despite having approved numerous other citizen-proposed designs. The next year, however, the justices ruled that the federal government couldn’t reject a trademark for an Asian rock band called The Slants on the grounds that it was ethnically derogatory. 

Breyer invoked The Slants case, known as Matal v. Tam, in the opinion for Shurtleff and his group Camp Constitution on Monday. “There, we held that trademarks were not government speech because the Patent and Trademark Office registered all manner of marks and normally did not consider their viewpoint, except occasionally to turn away marks it deemed ‘offensive,'” he wrote. “Boston’s come-one-come-all attitude — except, that is, for Camp Constitution’s religious flag — is similar.”

The question in all the cases is “Who is speaking?” If the government is speaking, then it can say (or not say) whatever it wants, but if it has created a public forum then free speech prevails. 

Shurtleff’s case drew some two dozen amicus briefs ranging from a consortium of 12 states to the American Civil Liberties Union, the American Legion, the Anti-Defamation League and the National Council of the Churches of Christ in the USA. 

That’s in part because the case could arguably affect numerous other controversial First Amendment issues ranging from campus “speech codes” to advertising on public transportation.   

In one of the amicus briefs, Rick Garnett, a professor at Notre Dame Law School, cited many other attempts by governments to limit religious speech. He pointed to Florida State University, which removed a member of the student senate for sharing religious views in text messages, to a high school in Washington state that punished a football coach for praying on the field after games, and to a Tampa transit authority that refused to let a synagogue advertise a show called “Chanukah on Ice” along a bus route. 

The government-speech rule is “an incredibly powerful doctrine” that can “cut off First Amendment claims at their knees,” said David Hudson, a professor at Belmont University College of Law in Nashville. 

Justice Gorsuch, joined by Justice Clarence Thomas, noted that one reason for the level of controversy is the common belief (including the belief held by Boston in this case) that the government can’t allow religious expression under the First Amendment, which is incorrect because the actual rule is that the government can’t prohibit religious expression under the First Amendment. 

“Not a single member of the Court seeks to defend Boston’s view that a municipal policy allowing all groups to fly their flags, secular and religious alike, would offend the Establishment Clause,” Gorsuch wrote. “How did the city get it so wrong?” (Gorsuch blamed some of the court’s own precedents for leading people astray.) 

The result of Monday’s ruling is that the doctrine will be a little less powerful. But it could tie governments’ hands in many cases and make them wary of allowing any speech at all if it means that they might have to open themselves up to speech they find offensive.  

“What the city cannot afford is the idea that the flagpole has become a place where … the Confederate flag, ISIS, al-Qaida, all of these could be flown,” Douglas Hallward-Driemeier of Ropes & Gray in Boston told the court during oral argument. “Or the New York Yankees,” he added. 

Breyer, who lived in the Boston area from 1967 to 1994, was sympathetic to that concern. “Boston could not easily congratulate the Red Sox on a victory were the city powerless to decline to simultaneously transmit the views of disappointed Yankees fans,” he noted sympathetically.