From my experience it's the young kids and new to 121 ex mil guys that haven't been shown how to jumpseat. My old regional took time out of ground school to explain how to jumpseat but that's probably not very common. On my current commute there has been one relatively new former military guy showing up on Southwest and just taking a seat and not saying anything to anyone. I think he learned a lesson one day when the flight filled up and he had to go up front.