I'm guessing we invented it, yes. Not all Americans like the same foods though.
I think when it started was my best friend's mom telling me about the time she worked in a ketchup factory. I was a kid, and she said that after cutting up tomatoes for some other product or something, they'd sweep up everything that was leftover that had gotten on the floor, and that was what ketchup was made of. Now, looking back as an adult, I know that's probably not true, but I still don't like the flavor of it. Granted, I also like to put salt and pepper on tomato slices, so I don't really like my tomatoes sweet.
I'm not a big hot dog fan, but when I do eat one I love mustard on them!