Faithfully executing the law

I wonder why the Constitutionally required oath of office for the President requires that he swear to "faithfully execute" the law of the land. Do you suppose it is because if the President were able to decide which laws he would or would not uphold might make it difficult to limit the power of the Executive branch? Imagine that Congress were to right a law that said that the President could not tap the communications of American citizens without the permission of the Judicial Branch. If the President did not faithfully uphold this law it would be impossible for anyone outside of the Executive to know about it. He refuses to get the permission of the Judiciary. He refuses to comply with information requests from the Legislative. Any case that comes up before the Judiciary by a potential victim is quashed because it would reveal State Secrets. The only way that any evidence would ever be forthcoming if a crime such as this were being committed would be in the format of an Impeachment Hearing whose main accusation was that the President was not faithfully executing the law as his oath compelled him to do.