On July 15, the FBI arrested the Russian citizen Maria Butina, claiming that she had failed to register as a foreign agent with the Department of Justice.
It seems to me that her activities in the USA were not illegal, the only problem was the lack of registration. Probably Ms. Butina did not know about that, or maybe the rules are not written very clearly.
Anyway, I think that it was much better, and cheaper for taxpayers, to just give her a warning before making an arrest. Or just fine and deport Butina, she is not the Russian Mata Hari.