I have worked in cafes and restaurants for many years. when i go in to them myself there are a lot of things that annoy me but this is definitely not one of them.
If someone goes to lengths to get my order right I would be happy rather than complain like you do. An Americano as you know is a black coffee. But people have it either black or with milk. If the barista always served it black without asking about the milk the percentage of people complaining and asking for milk after will generally outweigh the people wanting it black. It becomes very tedious and time consuming working like this. It is far better to be sure from the start.
I did my coffee training in Australia. Coffee is many years behind in England, and one of the problems is the customers dont know what they want. Some will order a coffee thinking it is a filter coffee and get a cappucino or americano or whatever.
In America before the stabucks era most people drank filter coffee. Not full milky coffee. That is where the term Americano came from. It means American style coffee, and it would not always have been black.
In Australia coffee is not explained in terms of America. There is cappucino, latte (in a glass), flat white, long black and long white.
A flat white is not a white americano as another poster said. It is 66% milk 33% coffee. Cappucino has 33% coffee, 33%milk, 33% froth.
Customers like their coffee in so many different ways. You should appreciate the barista going to the effort of getting your order right rather than moaning. They are probably saving time and complaints by working in that way, and satisfying everryone in the process.
Save your moaning for real coffee errors like burning the milk, burning the coffee (not scrubbing the machine every time), not being able to froth properly (creamy), or if it is too weak. Your complaint is ridiculous.