Say, for example, china wants to spy on a military contractor. Unless the NSA is sharing its secure pigeon network with every US defense contractor (and many of them, large and small) some pretty important US national security assets might be in play. So, perhaps not "state secrets" but things like technology inside of some tactical weapons guidance systems, or similar. The downside for the NSA of sharing any secret-pigeon networks would op-sec goes down as info dispersal goes up.

* Also for companies like a tesla or a space-x who may have purely industrial know how.