Great points! I've always wondered why or how a warrant could even be obtained on a peripheral entity when that person was never the intended target...and for the most part, anonymous until after the fact.
The Truong decision is an interesting case in that it highlights the distinction between "domestic" and "foreign" surveillance. Read on.
The difference between "domestic" and "foreign" national security intercepts, as the term is used by the courts and intelligence agencies, has nothing to do with the the point of origin, or the destination of the communication. Nor does it depend on whether either party is a U.S. citizen or resident. The difference between "foreign" and "domestic" national security intercepts is the source of the threat, i.e., whether at least one party to the communication was acting as an agent of a foreign power.
To highlight this, look at the Truong case. Truong, a U.S. resident alien, and Humphrey, a U.S. citizen and an employee of the USIA, conspired to commit espionage by delivering confidential government documents to the communist government of Vietnam from 1976 to 1977. Both were held by the court to be acting as agents of a foreign power, and therefore, a warrant was not required to conduct this national security intelligence gathering.
Note that in Truong, both the origin and destination of all calls the government intercepted were within the United States. Note also, that both defendants were U.S. residents, and in Humphrey's case, a U.S. citizen, as well. Yet the court held that these were foreign national security intercepts.