Nancy isn't even subject to it unless you think the phrase "authority of the United States" belongs only to one branch of government.
The drafters of the legislation drew it up quite artfully. They surrounded it with a lot of external "we'll bag those suckers" rhetoric in the propaganda pieces they had their friends in the press print for them, and then cleverly provided exceptions in the text that would make sure it never applied to any of them.
The "don't tattle on the agents" act that was supposedly violated when it was noted Valerie Plame was also Mrs. Wilson, is almost equally artful. It sounds like a really nasty law, and makes a lot of threats, but when you get right down to it the only guy it applied to was Aldrich Ames, and he was safely out of the country!
Andrew McCarthy was a Federal Prosecutor...so when he says she has broken the law under the Logan Act...I tend to believe him.