Why People Believe Americans Are Stupid

I have two similar situations when I worked as Ice cream man in USA.

1. I wore a t-shirt with label "London". Some guy look at the label and ask me:

Are you from France?

2. Two girls wants free ice cream from me. I tell them, OK I will give you free ice cream if you answer me a question. I asked them: "Where is Bulgaria?"
Answers:

In U.K.
In Australia
In Africa

But ofcource not all Americans are stupid, many of the smartest people are Americans.