I was looking at my cheap, in-a-carton coffee and saw “ミルクで簡単 おいしいカフェオレ” (Just add milk for a delicious café au lait.) written down the side. And I got curious. ミルク is milk. But there is a Japanese word. 牛乳. So why not that? A little bit of research and the general gist I got from Yahoo! Answers[ref]Clearly the ultimate source of language knowledge.[/ref] and it seems because ミルク is, like in English, milk so we commonly think of cow’s milk. But you could use soy milk, goat’s milk, or even people milk if you are into that. They are not constricting your use. No one can accuse these coffee dealers of being lactose normative.
The other idea is that since café au lait is not a Japanese thing, like coffee, the use of a loan word like milk helps to create a different feeling towards the product. We could sit here and argue if that is “racist”, “xenophobic”, or even “bad”, but on the same time we could be doing something valuable like drinking café au lait instead.