It's the whole "Is Pepsi fine?" Thing.

You ask if they have coke and they will say "is Pepsi fine?" Then you can say "Yes I'll take a Sprite" and get Starry.

Like you can't order a sprite without saying sprite. You cannot be like "I'll take a soda" and expect sprite. Same with saying coke. You have to elaborate.

You could ask for a "coke" and accept any soda. But you're not really calling what they are giving you coke. You just wanted any soda.