Those aren't the only two options! It's a cultural pattern. It doesn't apply on reflection -- if you're an Austrian soldier and Momčilo Gavrić points his gun at you, you shoot him -- but it comes up on the quick first pass.

Like tomatoes. Tomatoes are vegetables because they function as vegetables: you put them on sandwiches and in salads, and you don't eat them plain or put them in fruit salad. Then you think about it and realize that tomatoes are technically fruit. Or like that last sentence: tomatoes are the classic example of a vegetable that's actually a fruit, but come to think of it, cucumbers do the same thing, and sometimes you put apples or pears in salads...