The anti-ketchup crowd says ketchup and hot dogs just don't mix, period. Here's how Cecil Adams at Straight Dope puts it:
Ketchup smothers the flavor of the hot dog because ketchup makers add sugar to their products. That takes the edge off the highly acidic tomatoes, but it takes the edge off everything else, too. Which is exactly why a lot of parents like it, according to Mel Plotsky, sales manager for the David Berg hot dog company in Chicago. (Chicago is one of the hot dog's holy cities.) Put ketchup on it and a kid will swallow anything — and from there it's a straight shot to Velveeta cheese, Franco-American spaghetti, and Deborah Norville.
Others think hot dog elitists need to get off their high horses and let people eat how they want. This is America. People are free here, and that includes the freedom to put whatever you want on a hot dog.
Now, let me stop here and be clear about what I am not saying: I am not saying I think ketchup on a hot dog tastes good. I am a firm believer that hot dogs taste best when paired with excessive amounts of spicy mustard and sweet relish. This isn’t really even about ketchup. It’s bigger than that. What I am saying is that it is YOUR hot dog, and you should be — and feel — free to put whatever the hell you want on it. You prefer mustard only? Great! You like to get fancy and hit it with all sorts of exotic, non-traditional ingredients? Fun! You want to go all out for the holiday and bury it in chili, onions, and cheddar cheese sauce? Have at it, friend, although I may sit upwind from you for the next few hours, just to be safe. This is America. We believe in freedom, especially when it comes to high-fat meat products. Don’t let anyone intimidate you into believing otherwise.