They speak only English and expect everyone else to also.
Foreign languages are taught in American schools, but only as electives and second thoughts, not as mandatory curriculum. Because of this, and the fact that English is so prominent in their home country, Americans typically speak nothing else. Despite being the “Great American Melting Pot”, the United States is extremely large and Americans can be hard pressed to be exposed to other languages on a regular basis.
Also, word has gotten out that English is spoken all around the world. Though not always a first language, learning English in foreign countries is often required from a very young age. Almost all over the world you’ll find that English is spoken even if in small doses. Americans know this and therefore don’t work too hard on assimilating during their travels.