I don't think people were compensated for correct answers.
For the people taking the survey, selecting one answer had no more utility than another (except maybe if fucking with a survey can be considered fun). Therefore, I don't think that you call it stupidity for anyone to select one answer rather than another. It just didn't matter.
On the other hand, if you compensate people for the right answer, they start using the criteria of "what do I think the survey-maker thinks is the correct answer", which may not be the same as "what do I think is the correct answer".
A article in the Atlantic, linked to by someone on the unofficial LW IRC channel caught my eye. Nothing all that new for LessWrong readers, but still it is good to see any mention of such biases in mainstream media.
I break here to comment that I don't see why we would expect this to be so given the reality of academia.