ATF is a federal arm of the government.
The law Gregory broke was a D.C. Law.
Ignorance is no excuse.
It is if you're an Obama boot licker.
Let's see:
Article I §8: "Congress shall have the power...To exercise exclusive Legislation in all Cases whatsoever, over such District (not exceeding ten Miles square) as may, by Cession of particular States, and the Acceptance of Congress, become the Seat of the Government of the United States..."
So, in plain English, Congress' right to pass laws governing DC is EXCLUSIVE (no other entity can do it) and covers ALL CASES WHATSOEVER (including rifle magazines).
The "law" Gregory violated is no law at all.