If this is constitutional the First Amendment means nothing. From Natylie Baldwin at antiwar.com:
Establishment institutions usually start their implementation of censorship and the throttling of press freedom by going after individuals and outlets that are small and/or not well liked by a cross-section of the public. The obscurity or general unpalatable nature of the target ensures the success of setting the precedent.
Most Americans have probably never heard of Strategic Culture Foundation (SCF) and many would be unsympathetic once they learn that the U.S. government claims it is a think tank and journal associated with Russian military intelligence, although no evidence is offered to back up this assertion, and SCF denies it has ties to the Russian government. But years of anti-Russia sentiment in the political sphere and in most American media ensures that the claim alone will suffice to tar any Americans associated with Strategic Culture Foundation with a black brush.
I have confirmed that two American writers for SCF have received letters from the US Treasury Department in recent weeks warning them of fines of over $300,000 if they continue to write for the journal. These threats are in response to alleged Russian interference in the 2020 US election and part of the US government’s enforcement of Executive Order 13848 signed by President Donald Trump in September of 2018 which sought to ascertain foreign interference in any future elections in the US and to punish those deemed guilty. Threats to US elections included not only tampering with actual voting and its supporting infrastructure but “covert distribution of propaganda and disinformation.”