LogFAQs > #970373892

Usually because they like going out to eat and have reason to believe they can do so safely. Most restaurants are willing to accommodate allergies up to a reasonable point, provided the customer lets them know first. Sometimes, it's not possible or reasonable to accommodate the allergy (usually because the allergen is inextricably mixed in to the food or because the kitchen's too small to prevent cross-contamination), and in those cases the restaurant has a responsibility to refuse to serve the person with the allergy. Depending on where you are, certain common allergens must also be clearly indicated in the ingredient list (though that's more for packaged foods than restaurants).

Now, if staff weren't made aware of the allergy and didn't advertise their products as being free of the allergen, that's around where their responsibility ends. Pretty much everything is an allergen for somebody, and it's not realistic to expect restaurants to omit everything for fear of triggering a reaction. The customer needs to do their part for the restaurant to be able to keep them safe.