I’d think that this sign is not there because of American tourists. All the Americans I’ve met while traveling, have acknowledged that English being their native language is a privilege and have been very polite towards people who don’t speak English that well. But in Europe English has become the universal language and it’s easy to forget that not everyone can speak it as well.
I didn’t. The TL:DR of my response is that in my experience Americans don’t presume that everyone speaks English.