In calling on NFL owners and fans to punish athletes who engage in political protests, President Donald Trump has become a Super Bowl champion of something he purports to oppose: political correctness.

Apparently he’s fine with punishing dissenters, so long as he abhors what the dissenters are saying.

In recent years, many Republicans and conservatives have complained that political correctness — on university campuses, in workplaces and elsewhere — can squelch minority opinions and enforce a left-wing orthodoxy. They’re right.

What they mean is that if those in positions of power punish students, employees and others who dissent from the majority’s view, freedom is at risk, and society suffers.

If students know that they will be penalized if they say what they think — if they will be ridiculed or ostracized — they will just shut up. People won’t be able to learn from each other.

And note well: On one or more of those issues, those who silence themselves might even be right. So Trump was onto something when he said, “I think the big problem this country has is being politically correct.”

Like Trump, and many other Americans, I dislike it a lot when athletes refuse to stand for our national anthem. But our commitment to freedom of expression, and our rejection of mandatory conformity, can’t depend on whether we agree with the dissenters.

Call it the neutrality principle. As Justice Robert Jackson put it in 1943, “Compulsory unification of opinion achieves only the unanimity of the graveyard.”

In insisting that owners should fire players who do not stand, and in calling on fans to stop going to football games, Trump brazenly violated the neutrality principle. His words — “Fire or suspend!” - are more than a match for those of the most authoritarian university administrators.

To be sure, Trump’s supporters might point to an important qualification: As a matter of constitutional law, private employers are perfectly entitled to fire protesters. Americans often overlook the fact that the Constitution’s free speech principle applies only to government — federal, state and local officials — and not to the private sector.

That’s true and important. Those who run a business or a private university have a lot of room to maneuver. If you want your employees to be Republicans, or if you want your faculty to be left-of-center, you can do that, as far as the Constitution is concerned. The Constitution’s protection of free speech does not apply to the National Football League.

But because of crucial social norms, our freedom of speech, as we live it, extends far beyond constitutional law.

Countless employers give a strong message to their employees: If you do your job well, we don’t care for whom you vote, or whether you’re spending your spare time on protest activities. Those who run private hospitals do not inquire into the political views of nurses and doctors.

For private universities, the best practice is to insist that what matters is the quality of the argument, not whether it fits with the political convictions of the professor, the majority of the student body, the local community, or the president of the United States.

In a politically divided nation, professional sports are a unifying force. You can celebrate the achievements of Tom Brady, Walter Payton, LeBron James, Michael Jordan and Bill Russell without caring whether they agree with you about immigration, North Korea or climate change.

That’s good for sports. It’s good for fans. And it’s great for players, who know that they won’t be punished if they want to take advantage of a privilege that all Americans enjoy — to take some kind of stand.

Angry that some football players have not stood up for the national anthem, Trump complains of “the total disrespect certain players show to our country.” Maybe so. But to our freedom-loving country and its traditions, his graceless words — “Fire or suspend!” — are more disrespectful still.

Justice Jackson, that great opponent of political correctness, gets the last words: “If there is any fixed star in our constitutional constellation, it is that no official, high or petty, can prescribe what shall be orthodox in politics, nationalism, religion, or other matters of opinion.”

Cass R. Sunstein is a Bloomberg View columnist. He is the author of “ Republic: Divided Democracy in the Age of Social Media” and a co-author of “Nudge: Improving Decisions About Health, Wealth and Happiness.”