They were originally called Russian Mountains because they are descended from large winter sled slides popular in Russia, that became popular in Europe. When the first roller coasters were built in France, they called them "Les Montagnes Russes". I think many languages now call them Russian Mountains, although in North America, I've *never* heard them called this! They are called American mountains in Russia, because the first large roller coasters were built and popularized in the USA. So the history is tied to both countries! https://ru.wikipedia.org/wiki/Американские_горки

Russian Mountain? No, my friend, a roller coaster is a roller coaster here in the states. I'm curious who told you this.

Perhaps because they are an American invention? Good question