The people providing input into these textbooks are old Marxist cranks like Bill Ayers.
Sure enough, the garbage coming out of this year's AP government teacher is amazingly one-sided. (Palin's speech was written by someone else, she is not experienced, McCain shows lack of judgement in picking her, and other CNN talking points)
Why do teachers/professors become increasingly more liberal the higher one goes in education? There are plenty of conservative kindergarten teachers! Is it because the teachers with higher degrees are exposed to (brainwashed by) Marxist professors longer?