I can think of many reasons for restaurants to tell the truth regarding the items on their menus.
Food allergies are one obvious concern. People who have immediate and severe reactions when they eat certain foods need to be aware of the presence of those ingredients in the foods they are ordering. For some individuals, they may not even be able to be in the same room as foods that trigger reactions.
Some religions include dietary guidelines that believers must follow. When someone with such beliefs is traveling in a foreign culture, knowing the ingredients in unfamiliar dishes is the only way they have to remain within the requirements of their faith.
Health considerations are another reason for restaurants to provide truthful information about their products. Persons trying to limit cholesterol or sodium need to be aware of the amount of such items in the foods they eat. Persons who are working to limit or increase intake of certain nutrients appreciate guidelines to selection of foods that fulfill their needs.