I'm pretty sure that human activities ("artificial") are unnatural by definition. What justifies this is that we're humans, we have language, and we make words in those language to mean certain things (or make certain distinctions) that are important to us. So we can make the words mean whatever we want. We want "natural" to mean "unaltered/unaffected by humans", and "artificial" to mean "made by humans", so that's what they mean.

Are you looking for some kind of deity to define English words for you?