You can delegate authority but not responsibility. Holder is ultimately responsible for EVERYTHING that goes on in DOJ. If he did not know of an approved policy or "initiative" then he should have, otherwise he delegated without proper follow-up or supervision. An approving signature carries with it responsibility for whatever whether one asks questions or not. The head of a department is responsible for everything, for who is appointed, promoted, and whatever actions they take on behalf of the dept.
"The ultimate authority ... resides in the people alone. ... The advantage of being armed, which the Americans possess over the people of almost every other nation ... forms a barrier against the enterprises of ambition."
- James Madison