I am going to chime in here with my 2 cents. Having read through some of the posts, there are clearly some "preferences" as opposed to allergies or intolerances that people have described.
While I do not work in the industry, I do have a wife that has gone through some very tough years of medical appointments and cannot have a number of foods (that have been medically evaluated) While I understand that altering dishes requires extra work/attention. It has forced me to change how I look at food, prep, and cooking in general. As far as dairy goes, there are a number of things which can play into this category. Lactose intolerance is exactly that, the person has issues with the lactose in dairy. Simple enough. In some cases a person is ok with lactose, but has issues with the caseins which is why they can tolerate butter without problems, but not be able to have milk, cream, etc.
My wife falls into this category, so we have significantly changed how we cook in our house. That being said, I have eliminated a majority of dairy from my regular diet. Occasionally I will have a sweet treat or something that has dairy in it. That all said, I do a lot of running and have seen an amazing difference between the periods I consumed dairy and those which I dont. It has a significant inflamatory property to it that has reduced my recovery time and injury when not having it. (So for me it is a preference)
Likewise with gluten, there are a number of people who do this based on their preferences on how it makes them feel, others are actually celliac, and some are intolerant. The preferences are no different than the "no carb" diets, or the "no sugar no grain" etc etc, whatever the flavor of the month is. I get how this could be difficult to work with especially on some of the more exotic requests though.
As I said, there are described situations above which make you question the request etc. I love cooking, and again while I dont work in the industry, I enjoy the challenge of "recreating" or "crafting" something to accommodate certain requests (be it medical reasoning or personal preference) Obviously a heads up helps to prevent delays in sending it out.
my favourite one was getting an order for a crab salad,which hard to believe we served in the shell.waiter returns with the salad asking me to take it out of the shell and put it on the plate cause the customer has a shellfish allergy.when i told him that he couldnt eat the dish then he explained that it was not a severe allergy but he didnt want to push his luck.