So much for a quiet holiday week at Drexel University.

The Philadelphia college was rocked on Christmas Eve by a tweet from one of its professors, in which he was dreaming not of a white Christmas, but of a white massacre.

Ciccariello-Maher's tweets are private, but they've been reported by the Philadelphia Inquirer as saying "All I Want for Christmas is White Genocide."

A Drexel University professor is under fire for this tweet, which appeared on his personal account on Christmas Eve.

He reportedly followed this up with another tweet on Christmas Day, saying "To clarify: when the whites were massacred during the Haitian Revolution, that was a good thing indeed."

Reports on the tweet popped up on several conservative web sites, and the hashtag "White Genocide" was trending on Monday morning.

For most of this year, they’ve been making the case that the Harlem classic has been culturally appropriated. Now, Whole Foods is selling the sandwich for $8, prompting not just outrage over not just culinary appropriation but also “gentrification” and “Columbusing.”

The debate kicked off in February, when an Insider video featured the chopped cheese sandwich. Its white reporter extolled its affordability and ground-beefy, cheesy, oniony glory and bemoaned how “most New Yorkers don’t even know it exists.”

A Gallup poll in June 2015 found that almost 70% of U.S. millennials would be willing to vote for a socialist presidential candidate.

Millennials are one of history’s luckiest generations. We were fortunate to be born around the end of the Cold War a quarter century ago, when the tyrannical Communism embodied in the Soviet Union came tumbling down, also knocking socialism down a few pegs along the way. We have grown up in a world where, for the most part, economic and personal freedom are the rule rather than exception.

And apparently we hate it. How else does one explain why so many millennials seem to long to live in government-run economies, or worse?

A Gallup poll in June 2015 found that almost 70% of U.S. millennials would be willing to vote for a socialist presidential candidate. Even more shocking, a poll conducted before this year’s presidential election by the Victims of Communism Memorial Foundation found that barely half of millennials believe “Communism was or is a problem.”

The same poll found that a quarter of millennials hold favorable opinions of Vladimir Lenin, while 18% think favorably of Mao Zedong. More than 10% even have positive feelings about Joseph Stalin. Never mind that these men were responsible for the deaths of tens of millions and the impoverishment of hundreds of millions.

These polling numbers are frightening—especially when the Communist-ruled and socialist nations in the world today, from North Korea and Cuba to Venezuela, show so clearly how such systems invariably lead to repression and declining standards of living for their populations.

Part of the problem is that many millennials see these ideologies as represented by Scandinavian countries, an ignorant view fed them by candidate Bernie Sanders, among others. As Harvard and Stanford visiting professor Daniel Schatz (a Swede) wrote in Forbes in February, “Sweden began to reverse its economic model during the 1990s” through privatization and deregulation. Danish Prime Minister Lars Lokke Rasmussen was even more unequivocal in a speech earlier this year: “Denmark is far from a socialist planned economy. Denmark is a market economy.”

Scandinavian economies are in some ways freer than those in the U.S. The Heritage Foundation’s Index of Economic Freedom gives these countries high marks for limited regulatory burdens and for corporate tax rates lower than in the U.S. In many ways it’s easier to start a successful business and take part in economic life in a Scandinavian country than it is in America.

Millennials who wish to see a socialist or Communist Party-ruled nation in action should look to Venezuela, Cuba and North Korea. Venezuela’s current troubles make daily headlines. The country is crippled by inflation and shortages of basic goods, and the government takes more control over the economy each day. No wonder even millennials want to get out. A poll conducted by the Venezuelan market analysis firm DatinCorp in September found that 69% of youths there wanted to emigrate.

As Venezuelan blogger Maria Antonia Marturet recently observed, the country’s millennials are fleeing because, “They wanted to live in a place where they could go out at night without being kidnapped, where they could eat whatever they wanted without queuing, and where they could eventually have babies without the stress of not finding a hospital where to give birth.”

At least Venezuelan millennials still feel somewhat able to speak up. Communist Cuba arrests political dissidents. In North Korea, speaking against a regime that imposes starvation on its population can mean joining what the United Nations estimates are as many as 120,000 political prisoners kept in prison camps.

Young people living in Communist and socialist countries today don’t deserve U.S. millennials’ envy, but their concern and pity. There was nothing to admire about the Soviet Union, and there is even less to admire in countries that seek to perpetuate its failed philosophy at the expense of liberty and prosperity. Our generation is lucky it hasn’t learned this firsthand—and let’s hope we never have to.

Mr. Clark is the executive director of Generation Opportunity.