I think it is generally agreed that the AG has an obligation to defend a state's laws, particularly in a situation such as this where, under existing precedent, at least, the plan is not so obviously unconstitutional that no reasonable defense can be made. But this isn't a state's law - it's the University's admissions policy; and the Michigan AG is not the University's lawyer - the University operates separately from the state government. Granholm wants the AG to file an amicus brief on behalf of the Governor in favor of the University, and the AG is quite correct in refusing to do so if he believes, which he appears to believe, that the plan is unconstitutional. That is why the AG is elected in Michigan, rather than appointed by the governor - so that he can exercise independent judgment. He is not the Governor's lawyer.