Perhaps it where is the US you're situated. Granted, places in the southern US, especially those bordering Mexico will have more bilingual speakers. Several times when I was in Arizona and southern California, I had many people that came up to me and spoke Spanish, but never in the US mid west. Don't most schools in the US teach Spanish or is that an elective subject? I don't think that Spanish speakers would demand that everyone speak Spanish, but in bordering states, perhaps they have that unrealistic expectation.
I'm from western Canada, and even though Canada is a country with two official languages, most people outside Quebec speak English. There are French communities in western Canada that do have a French background, but I've never been approached by anyone expecting that I speak French. Now when I cross that border into Quebec, that expectation from the locals goes up considerably.
But I've never heard the words "How come you don't speak French/Spanish?" from anyone.
TazMan2000