The U.S. just reached 300 million estimated population not too long ago...
This is interesting in that yes Americans definately have a higher than average ignorance level.
However, keep in mind this....I guarantee you they did many more interviews than what are shown, and more people responded correctly. While it's definately true there are a lot of stupid people out there, and they deserve to be exploited to comic effect, but how can we know that this is a fair representation?
Another thing is something I learned in my communications class just today. And that is that most people in going into a conversation with someone, automatically assume that the other person is telling the truth and being honest. Now given this even subconcious preset, and the fact that this interviewer looks somewhat official, having a cameracrew and claiming to be someone of authority, it's not hard to see why someone could be easily tricked.
However...I don't mean to say Americans aren't stupid. Because really...how could you think Canadians have a bloody igloo as their capitol building?!