One more thing I just thought of that — since I have written or edited about a gazillion government briefs — should have been my first point. There is a legal presumption that acts of Congress are constitutional, one courts are supposed to indulge unless there is no plausible construction of the statute that would save it from constitutional error. Now, obviously, courts don’t seriously apply that — they invalidate lots of statutes for which a plausible defense can be made (even if that defense involves interpreting the statute in less than the most natural way). But, given that that is the controlling legal principle – at least in theory if not in observance — it makes sense that the Justice Department’s default position should be that statutes of Congress should be defended.