Is ketchup on a hot dog actually wrong?

The ketchup prohibition is a genuine Chicago tradition rather than a national culinary standard. The Chicago-style hot dog has a defined build, mustard, relish, onion, tomato, pickle spear, sport peppers, celery salt on a poppy seed bun, and ketchup is excluded from that specific construction, which local vendors enforce with real seriousness. Somewhere along the way this became repeated as a universal rule, helped by a widely circulated line from the National Hot Dog and Sausage Council suggesting adults skip ketchup, which was style guidance rather than a decree.

The broader point is worth making plainly: regional food rules are cultural identity markers, not correctness claims, and they exist across the map, no cheese on a Philly cheesesteak beyond the accepted options, no beans in Texas chili, cream in carbonara being a minor scandal in Rome. They are worth respecting when you are eating in the place that holds them, both as courtesy and because the traditional version is usually excellent for reasons the tradition encodes. Outside that context they carry no authority. Ketchup is tomatoes, vinegar, sugar, and salt, which is a defensible partner for a grilled sausage, and anyone telling you your lunch is wrong is expressing regional loyalty rather than reporting a fact.