I agree to a point, while it can be fun to discuss history, science, and what not, most of these things do not make up part of our daily lives, knowledge of algebra may not affect us in any way. I do think that most of us here could relearn that sixth grade algebra problem if we had to though, and with all the current events going on, there are a lot of folks opening their mouths that should be relearning some basics about our government.
An example, Bush and the domestic spying issue, how many folks out there are complaining about these policies infringing upon their rights, but they don't know what their rights are?
You have a good point about the sixth grade knowledge though, and this survey is only testing what people know off the tops of their heads. It could be possible that many Americans just grow up with this stuff and take their freedoms for granted.
Although I was born an American, I was not born in America, and did not move here until high school, and I am willing to bet that many immigrants who have moved here look at these things a little differently because they did not grow up with them, and did not learn them until they were older and thus they became more meaningful.