In 2014, residents of Washington D.C. voted to legalize marijuana. However, Congress quickly voted that the city could not use funds to implement the law. This was meant to prevent Washington from allowing recreational marijuana sales, but instead did the opposite: it created a virtually unregulated cannabis market.
Technically speaking, selling marijuana is illegal in Washington D.C. However, if you purchase an item that includes cannabis as a gift to go along with it, that's perfectly fine. And there's nothing that can really be done about it. Congress has prohibited Washington D.C. officials from implementing any rules, or even talk about implementing rules, that would be seen as helping to legalize marijuana in the city. So basically Congress has told the city they can't do anything about marijuana.
So the city has basically turned a blind eye to the situation. While the police have raided a few dispensaries that openly flouted the law, most have been allowed to operate without much interference.
The marijuana market in Washington D.C. is also quite large. It's the only place east of the Mississippi River that's legalized recreational cannabis. (Massachusetts voters approved a recreational marijuana initiative that will go into effect this summer.) So people from all around the East Coast travel to D.C. to purchase cannabis.
The big issue is the city isn't able to benefit from these sales. Because Congress says they can't implement any regulations, that also means they can't collect tax revenue from these sales. So the city gets all the benefits of legalization except for the one that would be most beneficial to the city.
It's possible that Jeff Sessions and the DOJ could decide to enact more strict policies when it comes to D.C. and marijuana. But for now, it'll just have to continue without the watchful eye of the government looming overhead.
(h/t LA Times)