As soon as the server makes that assumption, the customer ceases to be the asshole. At a restaurant in Frederick, Maryland, I worked with one cook named Chris who liked to claim that he had learned in “culinary school” that human beings could “only be allergic to like five or six things,” and anyone who claimed an allergy to something outside those boundaries was lying. The moral here is don’t be that kind of asshole, restaurant employees, because someday one of your coworkers might out you as a complete douchetart on a major national food website using your actual first name! Or you might seriously make somebody sick or dead. Either way, don’t be Chris. Chris sucks.
When I submitted this subject to my server friends, one of them used an excuse I’ve heard before but never even slightly bought: restaurants, when informed of an allergy, clean everything and use separate utensils and basically put on Cirque du Soleil in the kitchen in service of the customer’s issues. Look, I’m not going to say this NEVER happens. There probably do exist places that actually do this, somewhere, and if a kitchen is forced to decontaminate because a customer is full of shit, that customer deserves ire. But I worked in a lot of different restaurants, and I never saw ANY of them get particularly antsy even about cross-contamination, let alone the production described herein. It’s just not a thing that happens frequently (even assuming it happens at all). But if it does, it's probably for a very good reason: safety.