It is such a Chicago shibboleth that when Bob Schwartz, a VP of Vienna Beef, the leading Chicago wiener supplier, wrote a book, he titled it "Never Put Ketchup On A Hot Dog." He calls ketchup an "affliction."
Another famous Chicagoan, President Barack Obama, made it official with a proclamation on June 3, 2011 while visiting one of the Rudy's Hot Dog stands in Toledo, OH. Obama and the mayor stopped in for a chili dog, but apparently the subject of ketchup came up. "You shouldn't put ketchup on your hot dog," your President said.
So there you have it. Big government once again interfering on our right to chose.
The late great Pulitzer Prize winning Chicago columnist Mike Royko said it better than anyone in a column on November 21, 1995 "No, I won't condemn anyone for putting ketchup on a hot dog. This is the land of the free. And if someone wants to put ketchup on a hot dog and actually eat the awful thing, that is their right. It is also their right to put mayo or chocolate syrup or toenail clippings or cat hair on a hot dog. Sure, it would be disgusting and perverted, and they would be shaming themselves and their loved ones. But under our system of government, it is their right to be barbarians." This is not just a Chicago prejudice. In Detroit, the town that hates Chicago, Charley Marcuse vends hot dogs at Comerica Park for Tigers games. He is world famous for his operatic cry of "Ho-O-ot Do-O-ogs," a chant that delights and infuriates some fans. But if his siren song lures you in, don't ask for ketchup. He'll tell you in his tenor in no uncertain terms, "There is no ketchup in baseball!"
Nobody drove home the point more profoundly that Dirty Harry. In the film Sudden Impact, Clint Eastwood, playing San Francisco detective Harry Callahan, a.k.a. Dirty Harry, appearing at a crime scene, blows his top while watching a cop munching on a hot dog: "Nah, this stuff isn't getting to me, the shootings, the knifings, the beatings, old ladies being bashed in the head for their social security checks... Nah, that doesn't bother me. But you know what does bother me? You know what makes me really sick to my stomach? It's watching you stuff your face with those hotdogs. Nobody, I mean nobody puts ketchup on a hot dog."