I understand what you are saying.
But the defendant didn't do EVERYTHING he could do in order not to be breaking the law. He didn't study the laws of DC and he didn't avoid having the illegal magazines.
I'm not a lawyer and I am trying to understand under what circumstances a defendent is "not guilty" simply because he didn't know the law. In the last few days I have seen postings of the DC law and I don't recall seeing any mention of "intent".
Several years ago I travelled with firearms on a trip through the U.S., visiting about half of the states. Despite my best efforts I probably missed a few laws that could have gotten me in trouble. Would I have been "not guilty" because I didn't know, or just "not punished"?
There’s always a certain amount of judicial restraint and even jury nullification in such matters. The defendant clearly showed that he tried to comply with the law and a wise judge with no axe to grind will respect that.
When travelling through the US, you’re also protected by the 1986 Firearms Owner Protection Act (FOPA). This act protects you if the firearms are legal at the point of departure and the point of destination, even if you’re travelling through anti-gun jurisdictions. The firearms must be unloaded and inaccessible. Of course, states like NY and NJ routinely ignore this federal law and detain citizens for periods of time, then drop the charges.