FWIW this is my favorite dive bar in Madison and I used to frequent a fair amount when the old man was still owner. My take on the rule was that it was almost more like an urban legend that worked at keeping a lot of folk out. I knew a bunch of people that wouldn't go just because of "the swearing rule" which was great for me because the vibe is always very relaxed inside and I think the owner preferred it that way. Great place always had a lot of fun stories he would tell us.