USA vs Nixon being the relevant decision.
Exactly why Clinton was shocked.
Post-Nixon
Clinton administration
In 1998, President Bill Clinton became the first President since Nixon to assert executive privilege and lose in court, when a Federal judge ruled that Clinton aides could be called to testify in the Lewinsky scandal.[7]
Later, Clinton exercised a form of negotiated executive privilege when he agreed to testify before the grand jury called by Independent Counsel Kenneth Starr only after negotiating the terms under which he would appear. Declaring that “absolutely no one is above the law”, Starr said such a privilege “must give way” and evidence “must be turned over” to prosecutors if it is relevant to an investigation.
......USSC
.... “To read the Article II powers of the President as providing an absolute privilege as against a subpoena essential to enforcement of criminal statutes on no more than a generalized claim of the public interest in confidentiality of nonmilitary and nondiplomatic discussions would upset the constitutional balance of ‘a workable government’ and gravely impair the role of the courts under Article III.” Because Nixon had asserted only a generalized need for confidentiality, the Court held that the larger public interest in obtaining the truth in the context of a criminal prosecution took precedence.........