"For years, Boston has allowed private groups to request use of the flagpole to raise flags of their choosing. As part of this program, Boston approved hundreds of requests..."

"... to raise dozens of different flags. The city did not deny a single request to raise a flag until, in 2017, Harold Shurtleff, the director of a group called Camp Constitution, asked to fly a Christian flag. Boston refused. At that time, Boston admits, it had no written policy limiting use of the flagpole based on the content of a flag. The parties dispute whether, on these facts, Boston reserved the pole to fly flags that communicate governmental messages, or instead opened the flagpole for citizens to express their own views. If the former, Boston is free to choose the flags it flies without the constraints of the First Amendment’s Free Speech Clause. If the latter, the Free Speech Clause prevents Boston from refusing a flag based on its viewpoint. We conclude that, on balance, Boston did not make the raising and flying of private groups’ flags a form of government speech. That means, in turn, that Boston’s refusal to let Shurtleff and Camp Constitution raise their flag based on its religious viewpoint 'abridg[ed]' their 'freedom of speech.' U. S. Const., Amdt. I." 

Writes Justice Breyer, joined by Chief Justice Roberts and Justices Sotomayor, Kagan, Kavanaugh, and Barrett, in Shurtleff v. City of Boston, issued this morning. Justice Alito has a concurring opinion, joined by Justices Thomas and Gorsuch, and Justice Gorsuch has a concurring opinion that is joined by Justices Thomas and Alito. Justice Kavanaugh also has a concurring opinion.

You might wonder whether the Establishment Clause can justify viewpoint discrimination, but that's been dealt with in the past. That's why all the Justices agree: precedent. 

The text (at the link) includes this photo of the site of the flagpoles, Boston City Hall, which is ludicrously ugly:

 

Justice Alito doesn't want to analyze the problem in terms of government speech versus private speech. Looking at whether government is "controlling" the speech can cause courts to find "government speech" in the worst cases of censorship. The majority said "Our review is not mechanical; it is driven by a case’s context rather than the rote application of rigid factors." The Court’s "factorized approach," as Alito puts it, considers "history, the public’s perception of who is speaking, and the extent to which the government has exercised control over speech." 

And like any factorized analysis, this approach cannot provide a principled way of deciding cases. The Court’s analysis here proves the point. The Court concludes that two of the three factors—history and public perception—favor the City. But it nonetheless holds that the flag displays did not constitute government speech. Why these factors drop out of the analysis—or even do not justify a contrary conclusion—is left unsaid. This cannot be the right way to determine when governmental action is exempt from the First Amendment.

Justice Gorsuch writes to attack the old Establishment Clause doctrine known as the Lemon test:

Ultimately, Lemon devolved into a kind of children’s game. Start with a Christmas scene, a menorah, or a flag. Then pick your own “reasonable observer” avatar. In this game, the avatar’s default settings are lazy, uninformed about history, and not particularly inclined to legal research. His default mood is irritable. To play, expose your avatar to the display and ask for his reaction. How does he feel about it? Mind you: Don’t ask him whether the proposed display actually amounts to an establishment of religion. Just ask him if he feels it “endorses” religion. If so, game over.

Faced with such a malleable test, risk-averse local officials found themselves in an ironic bind. To avoid Establishment Clause liability, they sometimes felt they had to discriminate against religious speech and suppress religious exercises....

The Court hasn't used the Lemon test in 2 decades, Gorsuch notes, and yet the doctrine still intimidates some local officials into committing free-speech violations like the one in this case. And some local officials may be using it with an active desire to discriminate against religion. 

To justify a policy that discriminated against religion, Boston sought to drag Lemon once more from its grave... This Court long ago interred Lemon, and it is past time for local officials and lower courts to let it lie.