It’s a common and correct impression that Donald Trump is a man without principles, but he actually does have one immutable guideline: Anything that helps him, personally or politically, is good. Anything that hurts him is bad.

That simple precept explains Trump’s total reversal on the subject of TikTok, the wildly popular social media platform used by about 170 million Americans. When he was president, he called for banning the app, citing its ownership by a Chinese company, ByteDance, and exploiting fears of Beijing’s widening global influence. During his recent campaign, however, Trump realized TikTok could be an enormously useful political weapon and turned into its biggest champion. He might have made the switch out of crass cynicism, but he landed in the right place.

Congress passed a bill last year that will ban TikTok on Jan. 19 — unless its owners sell their interest before then. Trump has asked the Supreme Court to delay implementation of the law until after he becomes president. Others have attacked the measure as an unconstitutional abridgment of the First Amendment.

The court has agreed to hear the case on an emergency basis, and anyone who truly believes in free speech should hope the justices invalidate the law, or at least pause its impact and provide the new president a chance to negotiate an alternative settlement.

The reasoning behind the TikTok ban is rooted in two related fears: First, the app can gather, and transmit to China, dangerous amounts of information about American users. Second, China could utilize the platform to spread insidious propaganda that advances Beijing’s imperial aims. “The risks are real,” Trump asserted during his first term.

But proponents of a ban have failed to authenticate their arguments. TikTok, reports the Associated Press, “has long denied it could be used by Beijing to spy on or manipulate Americans. Its attorneys have accurately pointed out that the U.S. hasn’t provided evidence to show that the company handed over user data to the Chinese government, or manipulated content for Beijing’s benefit in the U.S.”

By opposing the TikTok ban, he also appealed directly to the young voters who monitor the app daily. “For all of those that want to save TikTok in America, vote for Trump,” he said in September.

There’s another dimension to Trump’s motivation. A generous campaign donor, Jeff Yass, is also a major stockholder in TikTok’s parent company, and he lobbied Trump heavily to protect the platform.

“It is hard to believe that Trump’s TikTok flip-flop is about anything much more than the influence of a billionaire donor and Trump’s reluctance to give up his access to his followers on the platform,” Democratic strategist Geoff Garin complained to The New York Times. “As with nearly everything Trump does, his change of position is driven by self-interest, not principle or the national interest.”

Garin’s right in one sense: Trump is always driven solely by self-interest. But he’s wrong in another. In this case, Trump’s personal profit led him to a position on TikTok that does serve the national interest.