First, this news source is heavily right leaning. For example:
The firearms laws in places such as the District of Columbia, Chicago, New York, Connecticut and New Jersey do nothing to reduce violence, but merely infringe on the Second Amendment rights of the law-abiding.
Random, biased statement in the middle of a news article with no sources, studies, or links to back it up. This is the equivalent to NBC stating during their nightly broadcast that Obama is the best president ever, like it's common knowledge. And then just going on to something else.
Not that this affects what is said in the article, just that you should take everything in it with a grain of salt.
Second, for those interested, Witaschek has quite a civil history. For a financial and wealth planner, he seems terrible at his job, given that he had a number of collection actions brought against him (including one by his estranged wife). He then ended up declaring bankruptcy 3 months ago (this is still ongoing).
Third, regarding the main issue here: The Supreme Court prevented a flat ban on certain weapons with their recent 2nd Amendment cases (specifically, Heller). The statute was modified in light of this to:
Spoiler
§ 7-2506.01. Persons permitted to possess ammunition.
(a) No person shall possess ammunition in the District of Columbia unless:
(1) He is a licensed dealer pursuant to subchapter IV of this unit;
(2) He is an officer, agent, or employee of the District of Columbia or the United States of America, on duty and acting within the scope of his duties when possessing such ammunition;
(3) He is the holder of the valid registration certificate for a firearm of the same gauge or caliber as the ammunition he possesses; except, that no such person shall possess one or more restricted pistol bullets; or
(4) He holds an ammunition collector's certificate on September 24, 1976; or
(5) He temporarily possesses ammunition while participating in a firearms training and safety class conducted by a firearms instructor.
(b) No person in the District shall possess, sell, or transfer any large capacity ammunition feeding device regardless of whether the device is attached to a firearm. For the purposes of this subsection, the term “large capacity ammunition feeding device” means a magazine, belt, drum, feed strip, or similar device that has a capacity of, or that can be readily restored or converted to accept, more than 10 rounds of ammunition. The term “large capacity ammunition feeding device” shall not include an attached tubular device designed to accept, and capable of operating only with, .22 caliber rimfire ammunition.
So now, you can have certain firearms, and ammunition. However, the ammunition you possess MUST fit a weapon you own. So here, owning ammunition without owning a compatible firearm is illegal under DC law. That is what this is about.
This hasn't been challenged under Federal law yet, which is interesting as NYC, Chicago, Illinois, and Massachusetts have basically the same law. So if this law is struck down in this case by SCOTUS, it should theoretically eliminate the other laws. But that's a few years away at least 
As for Witaschek, he has retained counsel, was arraigned, and entered a plea of not guilty on August 1st. A jury trial has been scheduled for November 4th.
