Interesting theory about helping recipients of large numbers of subpoena's streamline their responses / minimize workloads. Personally I think that's giving the government too much credit as if they cared about the burdensome nature of generating such large numbers of overreaching requests.
Good article, though I'd argue that the U.S. has considered foreign citizens not to be fully covered by Constitutional rights since way before 1990.
The DHS also claims that the 4th amendment doesn't apply to anyone within 100 miles of an international border - which includes a majority of the entire US population.
http://www.aclu.org/know-your-rights-constitution-free-zone-...
Edit: I appear to be misinformed about this.