Foreign propaganda is explicitly legal and protected in the United States. I'm sorry that you think it should not be, but it is, and it always has been throughout the entire history of the country.
How exactly do you intend to distinguish the types of speech that the government is allowed to ban? Any foreign ownership of a media organization from a country deemed an enemy of the state? And you're wondering about what needs to be said out loud?
That's not to say the United States Government cannot take _diplomatic action_ to counter propaganda. It certainly can, and it does. And indeed, the Congress just passed the Countering Foreign Propaganda Act a few years ago, to do precisely that in response to Russian efforts in the 2016 election.
But you know what it doesn't do? Ban the protected speech. We don't ban RT, which is broadcast into most of the homes in the country, despite being directly controlled by and funded by the Russian State, because that would be unconstitutional, and the Congress does not have the power to do it.
The standard in the United States is that the information published is both knowingly false and published with actual malice. And for matters of public concern, there is a further requirement that the information be provably false. Separately, the charges cannot be ridiculous, as demonstrated in Hustler v. Falwell, where it was held that even though Hustler published a story that Jerry Falwell had incestuous sex with his mother in an outhouse, and even though that story was false, known by the publisher to be false, and published with actual malice, it still did not meet the standard.
Misinformation campaigns are absolutely allowed in the United States. This is a current problem with Fake News. Fake news is protected by the First Amendment, no matter who writes it. And indeed, much of it is written by foreign agents, specifically with the intent of influencing US elections.
Second, the first amendment only applies to US citizens and those on US soil. So, as pointed out, American's have the right to seek out such information, but those oversees have no such right to have it disseminated here. Someone in the US can take the information and broadcast it on their behalf, which is how it happens now, but the notion that foreign agents have a right to create and disseminate whatever information they like is patently false.
How else are they going to disseminate media in the United States? They certainly could disseminate it via the Internet, but then the US would have no ability to block the information except via a Great Firewall, which the US does not have.
It's a distinction without a difference. RT is a Russian Government-owned propaganda network, with an American subsidiary, which operates in the US and broadcasts soft propaganda all day long.