This is not exactly true. RT does not freely operate in the US, it was forced in a court of law to register as a foreign agent. Legally the US government would have been able to arrest every single RT journalist beforehand and send them to jail for five years.
So yes, in court, it was decided whether or not RT was an agent of the Russian government, which is exactly the question at had in the post I was replying to.
This is not actually applied since the end of the Cold War, but legally per the Foreign Agents Registration Act if you don't register as a foreign agent despite being one, even without being notified you should, you are committing a crime can be convicted.
So, for example, being in contact with Xi Jinping or his lackeys and engaging in political action without registering as a foreign is actually a crime. And yes, people were prosecuted for that. But that's beyond even the point that I was stating, as a matter of FARA a court can and does inquire about that.
RT is not right now being pursued for it, but people in the US can be pursued for doing things such as lying or leaking national secrets, ie things that would normally be protected under the first amendment, if they are doing so in furtherance of the goals of a foreign government without registering officially beforehand.
For a recent case, see the case of Elliot Brodly which was found guilty for engaging in lobbying for a foreign government.
These laws in the US are very, very selectively enforced. The lack of prosecution of a certain entity doesn't mean anything. BBC reporters in the US are technically committing a federal offense by not registering as foreign agents, but the government will never prosecute them for that. RT, on the other hand, was forced to register, and WEB Dubois was prosecuted for even less on the same laws.