Actually, I don't think that's quite true. I believe that you have to be jump seat qualified per FAA regulations in order to sit in a jump seat. Of course, all pilots and flight attendants are jump seat qualified, but there may be other types of people that are (perhaps certain mechanics, for example). Among other things, people sitting in a jump seat have to be familiar with emergency procedures should an incident occur, so just being an airline employee isn't good enough.

I don't think you even need to be an airline employee to have jump seat qualification. Whether an airline would allow a qualified non-employee to ride jump seat is another issue entirely.