Re: But shirley...
The fundamental point here is that if the Feds need a warrant to examine the communications of a US citizen, then they will also need exactly the same warrant to examine the communications of an Iranian or Russian or Saudi or whatever other citizen in the same place and time.
It's called "equal protection of the laws", and it's guaranteed by the same constitution as the 4th amendment that everyone likes to bang on about.
You need to decide, are you going to allow this thing or not? One thing you can't do, however, is allow it "only against foreigners".