How is that illegal.
It's not.
The Treatment of Flynn’s Phone Calls Complies with FISA Minimization Procedures
By
David Kris
Tuesday, February 14, 2017, 5:02 PM
It is certainly true that U.S. intelligence services can get orders from the Foreign Intelligence Surveillance Court to monitor foreign officials. The Russian ambassador, simply by virtue of his nationality and official position, is an “agent of a foreign power” under FISA and hence a valid target for wiretapping. It is publicly known and acknowledged that the U.S. government uses FISA to wiretap foreign embassies and consulates. So, the Journal may be right that Flynn was picked up on a wiretap of the Russian ambassador.
So legal so far:
But a U.S. person’s name can be used when it is necessary to understand the foreign intelligence information in the report, and no serious argument can be made that Flynn’s identity was not necessary to understand the intelligence significance of his call with Ambassador Kislyak. The call is foreign intelligence information mainly because it involves Flynn.
In a related context, here is what FISA’s legislative history says about minimization rules and substitution of generic identifiers for U.S. person names when those U.S. persons are U.S. government officials (this passage is quoted in the chapter on minimization in my book on national security investigations and prosecutions):
One example [of a situation in which a U.S. person’s name could be disseminated in an intelligence report] would be the identity of a person who is the incumbent of an office of the executive branch of the U.S. Government having significant responsibility for the conduct of U.S. defense or foreign policy, such as the Secretary of State or the State Department country desk officer. The identifiers of such persons would frequently satisfy the “necessary to understand” requirement, especially when such person is referred to in the communications of foreign officials.
Flynn was not the incumbent at the time of the call, but the logic still applies. In short, this is not a close call.
Still legal. Flynn becane a national security risk because of his high national security position and he lied in public about his conversation with a Russian official. And Russians knew it and could use it.
Had Flynn not lied, it's likely nothing would of came from the legal wiretap on Amb Kislyak
although talking about sanction as a private citizen presented some kind of legal predicament.