Because the media is completely around the bend with their open hatred for President Bush. They want the TANG documents to be real, the "Bush joined the Guard to dodge service in Vietnam and then didn't even bother to fulfill that obligation" story to be real, and the whole revealing of the documents as fakes to be nothing more than a right-wing smear job perpetrated against the truth-seekers in the media by Karl Rove and White House to be accepted as proven fact. That proves, in their twisted minds, that all they and the left have said about Bush, about how he's pure evil, how he lied us into war to benefit his pals at Halliburton, how he stole the 2000 and 2004 elections, how his conservative base is worse than the Taliban, is really true.
So, since Mary Mapes echoes their thinking and their opinion, they happily put her on to rant and rave and slander the administration and conservatives in general, with no rebuttal or questioning of her claims.