It isn't just a union thing, its an Ops manual thing, signed off by the FAA. Taking unauthorized flight deck jumpseat riders just because you want to be cool, even if its "just some silly technicality" is a BFD. You are not only risking your ticket, but the ability of your airline to even have reciprocal jumpseat agreements. You are risking it for everyone just trying to be a laid back hero to one person that shouldn't even be there in the first place, and knows they shouldn't, but is there anyway just to see if they can put it on you to risk your job and your company's ability to have jumpseaters just so that they can get one free ride.