He does it because it is his responsibility as steward of the executive branch to retain the capacity of this administration, and every administration going forward, to function appropriately and independently from the congressional branch of government.
But there is a point beyond which it is his responsibility, as steward of the executive branch, to the American people and to future Presidents and future administrations, to retain the constitutionally enshrined separation of powers, to retain the capacity of the executive branch to deliberate and consult internally so that it can appropriately and independently respond to inquiries like this.
Norman Watt , lately Senior Steward, Edinburgh Castle, Scottish Executive (Edinburgh) Julie Bernadette Watters , health visitor (Wishaw, Lanarkshire) - For services to Health Visiting and to the Promotion of Breastfeeding in Lanarkshire.