I think whether or not airlines offer lounge access to domestic F pax is a function of (1) capacity and (2) profit. Many of CO's and NW's clubs are at capacity, especially in hub airports. If there is a large number of F pax who don't have lounge access via paid membership, elite status, or international J/F connection, admitting them might overflow the lounges. Of course their presence will crowd-out paying lounge members who would rather hang out for free in the airport than drop a few hundred to hang out in a jam-packed room with crappy food. Obviously, airlines would rather fill lounges with people who pay extra to be there.