To be fair, he also all but accuses NSA of interfering with US ally's Elections.
If it were confirmed, they'd have to switch to "Well if they weren't doing anything wrong they wouldn't have to worry about being interfered with, stop being ridiculous".
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FWIW, I personally would be relatively shocked if that(election tampering) did turn out to be legit. I suppose there's motive and means, but it just seems a little too far for the NSA. On the other hand, I can't understand why Snowden would put that in a tweet if he didn't want people to consider the possibility. So either,
1) He's a careless writer(I think this one is the least likely)
2) The NSA interferes with US ally's elections
3) Snowden is a Russian agent or has been coerced into that messaging
Also, like any organization, the NSA is not monolithic. It must have various strata, various different groups making it up. It is possible, for example, that 95% of the NSA operates in ways that are relatively ethical, but that there is also a more-or-less secret pipeline of information and control built into the organization's architecture that enables important figures both inside and outside of the organization to use its databases and tools and to coordinate its operations without the knowledge of most of its employees.