1) The intel guys always worry about “sources and methods.” The Flynn leaks remind the Russians that we are listening to every one of their calls. They expect it, but Americans were shocked that we listened to all of Angela Merkel’s calls, too. Is Russia the enemy, or do we just listen to everybody everywhere?
2) Before 9/11 and the Patriot Act there were very tight restrictions on listening to American citizens. NSA was forbidden, and if they got an American by accident they had to protect it from distribution. The FBI could tap Americans, but had to get a court order. The Patriot Act and the FISA court apparently made it easier for both agencies to listen in on Americans. .
But any tapping of Flynn as an American should have been subject to high level review and protection. The leaking exposes intelligence info to the Russians (presumably aiding the enemy) and probably violates Flynn’s 4th Amendment protection from searches and seizures. If they prosecute Flynn, this evidence may not be admissible in court, like the confessions obtained by torture at Guantanamo. Spying is a dirty business. It’s interesting that the media loves the 1st Amendment, but not the 4th.