I sometimes go to a place that serves Chicago hot dogs, a bit unusual considering that I live half a continent away. They have a big mass-produced sign saying that they won't put ketchup on hot dogs which for all I know is a national prejudice but one I always associate with Chicago. Maybe that's because the first time I encountered such an idea was in a Straight Dope column which originated from Chicago.
The thing that's so funny is that like so many prejudices the anti-ketchup people have a definite reason (ketchup is too sweet) but a reason that still won't convince anybody, maybe because it's arbitrary. Have you ever had a Chicago dog? Relish and a pickle spear? Peppers? Tomatoes? People that state "ketchup smothers the flavor of the hot dog" but who will eat a Chicago dog that smothers the flavor even more are simply dishonest.
These prejudices can be found in lots of places. I remember a Food Network chef saying that a well-done steak shouldn't even be eaten but really that's not his call. It's easy to start an argument in some places about the "proper" way to make chili or BBQ. Some people clearly enjoy questioning the authenticity of ethnic/international restaurants. I once read somebody who blasted all the supposedly ignorant diners who eat sushi with chopsticks (in Japan it's finger food) or who bite it in two instead of eating all at once, something that stood out to me because I hate eating sushi with my fingers and frequently the pieces are too big to really enjoy unless bitten in half. And people I know who've been to Thailand or Mexico then point out how our Thai and Mexican restaurants aren't quite like what is eaten there? Not news and even if it was nobody cares.
Now I know that much of this is just people creating arguments about something that doesn't matter or can't be resolved just as others will get into cats vs dogs, or DC vs Marvel, or pretty much anything involving sports. The difference with food tends to be that they can pretend to scientific reasons (the too-sweet ketchup approach) or historical ones (whether chili should have beans). In the end none of that matters. To paraphrase Duke Ellington if it tastes good then it is good.