Palin wont cooperate with the investigation because
1) she doesnt intend to get mired down by a partisan, smear infested "investigation" into a non issue
2) she doesn't intend to be the target of a politically motivated hit squad
I wouldn't do it either...until I could be assured that the people "investigating" were non partisan and had no ties to either campaign.
This is what we Aussies call "a kangaroo court". A more vivid term is "witch hunt".