Top secret and above is ONLY in the closed ecosystem of JWICS. There is no wired, satellite, WiFi, or bluetooth connection to any outside world network. There isn’t even a write DVD/blu-ray device or any USB ports on those machines.
Extreme acts of deliberate espionage, not reckless or careless acts, is the only means of delivering JWICS data to the outside world.
Then there is SIPRnet for FOUO, confidential and secret. Again, not means to get data from the CLOSED NETWORK of SIPRnet to unclas NIPRnet.
EXTREME ACTS OF ESPIONAGE!
Your point is correct, but just for details sake, FOUO can be processed in an approved NIPR system with PKI.