I think the wives actually went along on the ham radio group camping trips. I don't think it was a female-exempt thing. Just that Carol was not Russell's wife.
Over the years, the pair had camped together regularly with their wives and other radio club members.
“I think they’ve been murdered”
ETA: The weird thing is this article says
“Carol was never ever brought on radio club trips. And apparently Carol’s been around the scene for 20 years, on and off,” Mr Ashlin said.
But I am sure that later on (or earlier) Rob did say that Carol had been on one of these trips, and others didn't like it. Maybe I will stumble on that link again at some point.