It is true that Butina repeatedly told him she wasn’t a spy and Byrne ultimately became convinced that was true. But even in his description of that, he told Carter that he believed Butina was being used by US and Russian intelligence, not that he believed she had no tie to intelligence.
...
If that’s what he told the FBI, it does nothing to make her any less of an unregistered agent of Russia.

LOL, so being a "useful idiot" means you are guilty of being an unregistered agent? Guess that's the state of the "law" in the USA these days.