The Springfield News-Leader explains, briefly, the Federal stance on legal marijuana use prior to the election:
Yes. It’s still illegal under federal law. Federal officials currently serving in the Trump administration, such as Attorney General Jeff Sessions, have opposed marijuana reform in general.
Now that it’s passed, however, the Springfield News-Leader wants to make sure that those who violate Federal law obey Federal law:
Medical marijuana may have been legalized in Missouri, but those who opt to take advantage will be jeopardizing their Second Amendment right to buy and possess a gun.
Under federal law, Missouri residents won’t legally be able to have a license for medical marijuana and possess a firearm at the same time, even though voters overwhelmingly added Amendment 2 to the Missouri Constitution on Tuesday.
The article is actually a pretty good exploration of the intersection of the two and the current law enforcement climate and not just an exhortation to give up your guns.
But I certainly didn’t see a full article about how the state measure violated Federal law before the vote.