Because cross contamination. If you are allergic to peanuts ask about the fry oil because it could be peanut oil. That's why you specify an allergy. Not "I don't want tomato's on my burger" even thought you allergic and they use the same cutting board for lettuce and tomatoes.No dog in this race but if I’m putting in a food/drink order and I specifically say I don’t want a specific ingredient in my order that means I don’t what that specific ingredient in my order. That should be the end of conversation.
Why should I have to justify why I don’t want that particular ingredient?
That’s like having to explain to the a server you don’t want cheese on your burger because you are on a diet.
Again just saying do not put something is not enough.