I'd also imagine that it's far easier to just go after the sources (whoever is allegedly forking over the data). Not to mention that it's probably much better organized at the source than in some sort of data dump format. Just a guess, though.
Tell me when the NSA, or the US in general pre-NSA, has ever endangered or compromised their encryptions? There surely is enough stories of how the CIA screwed up and stories like the FBI helping Whitey Bulger to continue murdering while he was an FBI informant.