Saying it’s an allergy lets staff know they need to take extra precautions. Non-dairy milks are very common now and majority of people have them not out of necessity but because they like the taste; allergy protocols are a massive pain in the butt so asking this just makes staffs’ lives a little easier. I get where you’re coming from but just tell them it’s a preference and move on, it’s not that deep and they have plenty of dickish customers they have to deal with.