"If I have a pile of NSL's and I count them: 1...2...3...4...5...6... and then I publish that there are '6' NSL's, well then...
"Each of those NSL's is part of that count. And if you just take a properly charged magic wand and wave it over that '6' then you can scry each of the individual NSL's that made up that count.
"As you can clearly see, that would expose the contents of each of the 6 NSL's. So we must take care to keep that '6' confidential, just as we keep the NSL's confidential. That '6' is six times as secret as each NSL!
Suppose, for discussion, that the defense is short of time, money, or brains: in which case the agent-of-government may not even have come up.
So from the prosecution perspective: better not to mention it and hope it never comes up. This is how citizen rights are "innocently" violated wholesale, and it happens all the time.
Worse, it only came up because the Best Buy agent was paid. Suppose a different scenario: the Best Buy agent was cooperating to avoid prosecution for something they did. Well, that makes it all better, doesn't it, to have them directed by the FBI and no money changing hands--and also much harder for the defense to prove.
The problem is, first, the direction of a confidential agent by the FBI (whether paid, quid pro quo, or volunteer) and, second, the FBI's (repeatedly demonstrated) willingness to conceal the fact that the agent was under FBI direction.
The bottom line was that the FBI hoped it would remain unnoticed that such a relationship existed. As they see it, just bad luck that this time it got found out. Better luck next case.
IMSI catchers are getting so common it's getting to where you're more likely to connect to one of those than to a regular cell tower.
Maybe we're thinking about this backwards.
Maybe the proper approach is to simply order law enforcement (i.e. government) to operate the entire cell network. Yeah, no privacy, but that ship has apparently sailed already. Certainly Congress isn't going to help--three guesses what any IMSI catcher law is going to do for privacy: nothing, nothing and nothing.
But think of the benefits: Coverage would improve. No more roaming. No more "can you hear me now" shill commercials. No more data caps. Probably many more.