Actually, they not only can but should do this. The NCAA can't force ANYONE to cooperate but they can and should threaten the player with retroactive ineligibility for not cooperating. The evidence is already there that he cheated. That is why he sat out the remainder of the season.
An even better example is Maggette. He admitted to lying and cheating. The NCAA exonerated him and Duke of all wrongdoing.
If you don't think there is favortism, explain Maggette to me!
Maybe you're right, but when has even the NCAA threatened a player with retroactive ineligibility for not cooperating?