From the National Review, “Why Yates Had to Go“.
It is a very simple proposition. Our Constitution vests all executive power — not some of it, all of it — in the president of the United States. Executive-branch officials do not have their own power. They are delegated by the president to execute his power. If they object to the president’s policies, their choice is clear: salute and enforce the president’s directives, or honorably resign. There is no third way.
No one knows this better than high-ranking officials of the Department of Justice. That is why President Trump was right to fire Acting Attorney General Sally Yates.