You might want to examine the underpinnings of your desire to find fault with Americans since this kind of comment raises much more questions about your psychology than it does about Americans.
The fact is that this joke sometimes even temporarily fools people who are familiar with water's molecular make-up (and yes they do teach this in every American high school) because most common substances are not referred to by their chemical name.
I have to admit that especially in Germany there is a huge, huge bias (from our news media) towards pointing the USA as a nation of radical right-wing retards.
US-related news here is mainly about: Tea Party, gun violence, wars, more Tea Party, corruption and dumbness on both popular parties, sheer political idiocy (sequester), and a record amount of bigotry throughout the society.
To the USA: deal with your external image before starting to whine when other people laugh at you.
The fact is that this joke sometimes even temporarily fools people who are familiar with water's molecular make-up (and yes they do teach this in every American high school) because most common substances are not referred to by their chemical name.