A little known fact is that the Justice Department is required to defend Congress's laws. Congress put the import restriction provisions into the law, and the Justice Department is required to argue in favor of them.
People love to say this, but nobody has ever been able to show me anything supporting this idea that they are 'required to' defend anything. Please cite the legal authority behind this statement.
(As an example: they're not defending DOMA, which was Congress' law.)
More generally, the Department of Justice is the Attorney General's office, and the Attorney General is the Government's lawyer. Congress is the primary organ of government. Like any lawyer, the Attorney General has a responsibility to vigorously represent his or her client except to the extent it's illegal (unconstitutionality is illegality). This includes arguing for interpretations of the law that favor the client.
Sorry, I don't buy this line of reasoning. The President's (and, by extension, the whole Executive branch's) duty is to 'faithfully execute' the laws.
I can't see how a rational person would read this clause as pushing some sort of obligation on the Executive to defend laws against challenges in the judicial system. That moves beyond 'execution' by several steps.
(That said, I appreciate the pointer and can see how people could interpret the clause in that way. I disagree that it is a correct interpretation, but there's plenty of room for different opinions.)
It goes back to a lawyer's duty to prosecute his case fully. I'm in violation of my obligations if I don't argue for any justifiable interpretation of the law that might favor my client--I don't get to not do so just because I don't think that interpretation is the correct one. It's not my job to pick the correct interpretation, that's the court's job. That's pretty much how the DOJ views their obligations to Congress.
