What's the most stupid of all about this is that ordering a burger without ketchup a is actually rather populist in many respects.
http://newmajority.com/ShowScroll.aspx?ID=deee68fd-d510-43d1-8b52-656f064b0aa6Texans traditionally eat hamburgers with mustard or with mayonnaise (or with both), but without ketchup. This is simply called a “hamburger” in Texas, but is sometimes called a “Cowboy Burger” or a “Texas Burger” outside of Texas.
A hamburger with ketchup is sometimes called a “Yankee Burger.” A hamburger with mayonnaise is sometimes called a “Sissy Burger.”
Dirty Martin’s (in Austin since 1926) serves hamburgers with mustard, pickles, onions, and tomatoes, but it is not known when this combination began. The popular Texas “Whataburger” hamburger chain has served hamburgers with mustard from its founding (1950). The hamburger-with-mustard combination in Texas is attested at least from the 1950s, but the pre-1950s hamburger condiments cannot be firmly established.
Second answer: Republicans. A 2000 survey of members of Congress by the National Hot Dog Council found that 73% of Republican lawmakers preferred mustard to ketchup, as opposed to 47% of Democratic lawmakers.
Final answer: traditionalists. Louis' Lunch in New Haven, Connecticut, the restaurant widely believed to have served the first hamburgers ever made in the United States, absolutely forbids ketchup.