I'm not convinced this accurately reflects the capabilities of law enforcement or the enforcement of laws on international entities. GDPR, for example, is a regulation that applies to non-EU companies when they operate in the EU. I don't believe it's accurate, then, to say that it's impossible for a country to apply regulation to foreign entities running web services.
Additionally, I'm not convinced that the US is actively investigating such things, at least formally, on domestic social networks anyway. Twitter's algorithm at some point was tweaked to amplify Musk's posts. Was there a formal investigation or certification process, or were any controls put in place, to ensure that nothing Musk Tweeted would ever be subject to foreign influence, and thus enable amplification of foreign propaganda or ideology? I quite doubt it.
I didn't think Twitter was even required to inform the government that his Tweets were being boosted. Researchers discovered it. (And I think there were some leaks from Twitter developers, via journalists.)
It just seems weird that we seem to put very minimal effort into verifying that domestic networks aren't pushing foreign propaganda, but then our logic was excluding foreign social networks is that they might push propaganda. We don't even know that the American ones aren't, and we don't even seem to care very much.
They can't do that with offices that are parties offshore, owned by non-Americans.