I don't think it is anything like that. It's for cases where they need to discuss something that they don't want every person with a HAM radio to hear. A discussion between an astronaut and the flight surgeon/spouse/clergy member. I'm sure the Russians have a system on ISS as well. Just having the hardware is not enough to crack into an enemies comms. It's more that they don't want stuff like that falling into the hands of a rouge state like Iran or North Korea, or a non-state actor that might have schemes of reverse engineering it to build their own system. I'd bet there is a Russian Transceiver racked up in JSC, and a DOD system racked up at Russian Space Control.