Why the NSA rather then DARPA or some other government agency? Is it just because the NSA are rather unpopular at the moment, so the halo effect leads people to decide they might destroy the world?
Why the NSA
Because they have a great deal of resources and very little oversight.
The cryptocurrency ethereum is mentioned here occasionally, and I'm not surprised to see an overlap in interests from that sphere. Vitalik Buterin has recently published a blog post discussing some ideas regarding how smart contracts can be used to enforce superrationality in the real world, and which cases those actually are.