What exactly is the distinction you see there?
1. People in general who have opinions that might happen to align with a foreign power's stance.
2. People who are literally "agents" of a foreign power as defined under laws like the Foreign Agents Registration Act of 1938 [0], which often involves being directed/hired/paid.
That'd be a significantly different legal standard. (The constitutionality of FARA-type laws being a separate question.)
[0] https://en.wikipedia.org/wiki/Foreign_Agents_Registration_Ac...
Only a truly naïve person would not have seen this next logical step (prosecutory threats) coming when the preceding step received virtually no opposition.