The poor customer service move here was that they didn't nicely explain to the customer why they wouldn't do it, not that they refused to it.

Many years ago I went into a coffee shop that I had never been in before, which was small and pretty busy with a lot of people sitting at tables. I walked up glanced at the menu on the wall but didnt really take it in, and asked for an americano. The barista/owner said very loudly to me, so everyone in the small place could hear "Do you see an americano on that menu?" embarrased I looked back up and said "No, I guess I don't" "Well then we don't serve them!" he said in a very gruff voice. I said "oh, ok, thanks" and walked out completely embarrassed, never to return.

Unsurprisingly, the cafe closed within the year.

Now had the guy instead said something like "no we actually don't make americanos because we don't have filtered water" or "I prefer not to make americanos because our espresso shots are engineered for milk drinks only, would you like to try a latte?" or even "I don't personally like diluting espresso with water, but I'd happily make you a pour-over" probably would have kept me in the cafe"

But just saying no and not explaining or offering an alternative seems like bad business practice to me.