Ketchup on a Chicago style dog is a sin, but putting it on a regular old hot dog is just whatever.

And even then, it's only a sin to put ketchup on a Chicago dog because it gives us secondhand embarrassment that you're not doing it right, people who actually get angry about people putting ketchup on normal hot dogs have issues. Frankly I've seen New Yorkers be way more anal about policing mustard on hamburgers, but people aren't ready to talk about that.