In this case: "allergies and intolerances are made up stuff for weaklings, haha".
It's kind of similar to the Whole Wheat Bakery asking you whether you're OK with gluten. If you aren't, you made a big mistake walking in.
Affluent areas in general have more variety. The ice cream shop may be a place where you can get all kinds of ingredients that you wouldn't find at other places. This is 100% true for "fine dining" and it's one reason why they ask.
They will also have substitutes for an allergy to make the experience just as pleasant, thus they ask.
> Affluent areas in general have more variety.
No, it's definitely a difference in cultural norms, not something driven by the store inventory.
> They will also have substitutes for an allergy to make the experience just as pleasant
This is not the case for dairy in an ice cream shop, or for wheat in a "whole wheat bakery".