The New York Times has a story about restaurants that refuse easily-accommodated customer requests:

At a pea-sized Lower East Side bistro known for its fries, the admonition is spelled out on a chalkboard: no ketchup. At a popular gastropub in the West Village, customers cannot have the burger with any cheese other than Roquefort.
And at Murray’s Bagels in Greenwich Village and Chelsea, the morning crowd can order its bagels topped any number of ways but never — ever! — toasted. “It’s really annoying, because a toasted bagel is kind of fierce, right?” Jamie Divine, a product designer and frequent patron, said with a hint of an eye-roll.

Matthew Yglesias describes this as specialization:

What I don’t understand is why she sees this as some kind of ethical stance, a form of Puritanism. It just seems like specialization. In any line of business you face a tension between the fact that it’s more efficient to specialize but you have more customers if you customize.
David Chang explains it perfectly:

“People just assume that every restaurant should be for everyone — I could understand that if we were in a town with, like, 20 restaurants,”said David Chang, whose mini-empire of Momofuku restaurants is well known for refusing to make substitutions or provide vegetarian options. “Instead of trying to make a menu that’s for everyone, let’s make a menu that works best for what we want to do.”

I don't think that's it. The cost of providing a bottle of ketchup at a restaurant that specializes in fries is very low. The point of this is to signal culinary sophistication to customers. This is the kind of restaurant that refuses ketchup to fry-eaters. It sends a message about the kind of food served and the kinds of customers who eat there. They're selling cultural exclusivity.