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.