The answer is that it doesn't surprise them a bit, and for the left, this should be a "dog bites man" kind of story, but the report it anyways simply because it fits, and reinforces the template.
Likewise, the MSM has painted Obama as a man of letters, and far and away the most literary of all presidents. The fact that Ayers essentially wrote his book, while newsworthy in a "man bites dog" kind of way, does not fit the media template, and consequently it goes unreported.
If Obama and his war room and the MSM did not have so much invested in the “dolt and buffoon” Palin narrative and in this case Time Magazine, they would not be putting themselves in an untenable position consistently to prove its veracity.
They know that if Sarah’s political and intellectual credentials were ever LEGITIMIZED in the mind of the public, Obama would lose the next election. Thus the constant barrage of innuendo, smears and mockery to DELEGITIMIZE Sarah which is also reinforced by elitists and media pundits.