Permit me to rephrase it. Shouldn’t the same thing be said of defending the law?
The government passes the law and then must defend it. It is the citizens that challenge laws they deem unconstitutional.
No, absolutely not.
Each branch must determine the constitutionality of its own particular actions, on its own.
There is absolutely nothing in Art III that sets up an American Cour de Cassation like they have in France. The USSC "power" of judicial review, correctly applied, extends only to cases before the court where a violation of a law is at hand. THEN, the Court must decide (for itself) whether a decision would enforce a law which is not a law because it violates the Constitution.
The Congress and the President have analogous powers in their own spheres.
The President cannot fail to enforce an Act of Congress signed by a President, but he can in fact, he must, decide the scope of work of the Justice Department - that's his right, and his responsibility.