There are some old-style coax BNC connections that MUST have termination caps in order to perform properly. I had some old lab instrumentation that ran coax back to a coax/ethernet hub, then connected to the computer via ethernet. This instrument had 8 discrete modules all daisy-chained together via coax. And, the end of the daisy-chain absolutely required the proper termination cap.
So, I suspect that these RCA caps are trying to play on that to cheat people out of there $.