There needs to be a law against what they did before they could be indicted for anything. Afaik there is no law against being a foreign propagandist.
Even the two handlers themselves would have been fully legal if they had simply registered as foreign agents.
Our first amendment protects these things, for better or for worse. It protects the right to lobby the government (petition for redress in the official language), with no bar to people doing it on behalf of foreign governments, which is why all we do is make them register under FARA for transparency. We’ve lived under this legal system through the whole Cold War.
Speech is similarly protected, even if it is at the behest of foreign governments.
Our first amendment protects lies and propaganda just the same as everything else, which is why any of us can still go look at RT right now if we wanted. If we can’t even ban RT with all the sanctions we have on Russia right now, how the hell are we supposed to go after these American citizens?