Lots of money? If you can short cut mining, it's cleaner black money than dealing drugs or people, probably. And if you can steal from someone else's address, maybe that's even better... You can seize funds with no red tape. Or mete out that capability to allies etc. Why wouldn't the NSA want to support a parallel financial system that criminals flock to? Just like why wouldn't the NSA want to covertly support encrypted messaging apps or the FBI do that...