The administration says that releasing them would inhibit the Pres from getting candid advice and sets a bad precedent that would damage future presidents ability to get advice.
Unless a president can protect the privacy of the advice he gets, he cannot get the advice he needs.
Under the doctrine of separation of powers, the manner in which the president personally exercises his assigned executive powers is not subject to questioning by another branch of government.
Oops, I quoted the wrong president. Those words didn't come from Crusader Bunnypants, they came from that other shining example of the Republican Party;
Richard M. Nixon.