Actually, they kind of are.
There are no cases directly on point, and they are arguing by analogy to existing cases where it was held retrieving small amounts of phone data about one or two individuals did not violate a reasonable expectation of privacy, and thus, not a search
This is not the same as "what they are doing is permitted by the current laws of the united states".
I'm aware folks like Orin Kerr think this doesn't make a doctrinal difference: http://www.volokh.com/2013/07/17/metadata-the-nsa-and-the-fo...
The thing is, he's an academic (prior to his current job, his job at the DOJ CCIP division was being an expert in those notices you see when you access government computer systems). He likes to think in academic terms about whether things should matter. For example, he also believed the "mosaic theory" was a bunch of bullshit and would be struck down by every level right up until the supreme court fully endorsed it (see, e.g, http://www.volokh.com/2010/08/06/d-c-circuit-introduces-mosa... and other posts)
So while i certainly trust his view of "the state of the caselaw as it currently exists", i take his view of "what a court is likely to decide" with a huge grain of salt. On that front, IMHO, he's been wrong more than he's been right.
For example, when he says "If obtaining pen register information on one user is not a search, the obtaining that pen register information for 100 or 10,000 or 1,000,000 or more users is still not a search. Katz tells us that the Fourth Amendment protects “people, not places,” and it’s not clear how surveillance that is not a search when provides information about one person can become a search when it provides information about many."
This tells you everything you need to know. It is a typical ivory tower viewpoint.
He doesn't see how a judge is going to decide that there is some line that has been crossed when you do something to a million people instead of 1. As the saying goes, "bad facts make bad law". If everything was doctrinal and academic, bad facts wouldn't matter.
FWIW: The FISA authorization is completely and totally irrelevant in this case. Entirely. The fact that the FISA court said "sure, whatever" does not make something legal, only authorized, because they are not ruling on a constitutional challenge, only the validity of a search warrant. These are not the same, no matter how much the government wants it to be.
This is the same as a valid search warrant that later turns out to have been facially invalid; It is not a legal search warrant that has "gone bad". It is illegal. It was authorized, but illegal.