Jeff Sessions allows the U.S. Attorneys to prosecute the Federal Marijuana laws. . .and that's a really good thing. . .

in spite of what all the weedy stoners and now drugy states may state about it.

Sessions didn't write the law.  He's not even requiring that it be enforced.  He's merely withdrawn the Obama era declaration that U.S. Attorneys violate their oath to uphold the laws of the United States and abstain from prosecuting for the violation of a Federal law, at least one of two instances in which President Obama, unable to secure a repeal of a law, mandated that his officers violate their oaths of office and fail to uphold it.

Now, it's no doubt clear from my posts here I'm not in favor of legalizing marijuana.  Frankly, while I'm not a teetotaler, I wouldn't have been in favor of the repeal of Prohibition either.  The evidence is good enough that humans fall into destructive addictions easily, and that taking any drug isn't really a good idea if you can avoid it.  I've already gone into the topic of why alcohol is both destructive and perhaps a bit of an exception from an evolutionary biological prospective, and if people want to read and complain about that in that thread, hey, have at it.

"Radar plot depicting the data presented in Nutt, David, Leslie A King, William Saulsbury, Colin Blakemore. "Development of a rational scale to assess the harm of drugs of potential misuse" The Lancet 2007; 369:1047-1053. PMID:17382831. For more information, see image. It contains not only the physical harm and dependence data like the aforementioned image, but also the mean social harm of each drug. This image was produced with the python plotting library matplotlib"  Note, alcohol is on the high side of social harm in this plot, but then it would be.  It's the only legal drug in the lot and therefore the most used by default.

This topic isn't on any of that.

Having a law on the books breeds contempt for the law, particularly a law which at least a significant minority of a population supports.  If the law is no longer wanted, it should be repealed.

Now, critics will scoff and say this is an exception and it hasn't been enforced for a long time, etc. They're wrong.

And the reason they're wrong is not only that ignoring a law breeds contempt of all law, but it sets a precedent.

Don't think so?

Well, consider this.  This law was ordered to be ignored as the Obama Administration basically is okay with states going their own way on marijuana, but Congress wasn't.  So the order went out, ignore the law.

How many other closely debated topics are there like this?

Plenty.

Some examples.

Some right wing politicians have long argued that Form 4473s which people fill out when they purchase a firearm are excessively intrusive.  I disagree, but they argue it.  Others argue that prohibitions on any type of weapon, such as machineguns, are improper and illegal. What if some administration, say the Trump Administration, ordered the Federal government to cease all prosecution of Federal firearms violations?  Would states like California be cool with that?

 M249 automatic rifle (it's not a machinegun, silly). You'd be okay with the Administration ordering no restrictions be enforced on these. . .right?

In my state there are two Gubernatorial candidates who are already arguing that the Federal government and its nasty regulations are holding the state down, even though the Trump Administration in fact has cut regulations like weeds with a weed whacker. Frankly, the argument isn't credible at this point, if it ever was, but what if Trump simply ordered that no environmental regulation be enforced.  None.  Would that be okay by you?  It wouldn't be with everybody.  If that example sounds extreme, I could frankly easily see him doing that.

And something like that has in fact happened before. At one time, under one GOP Administration, the Federal government simply didn't budget for MSHA (Mine Safety) training.  The regulations said that the training had to take place, and it had to be from a certified MSHA instructor, but the Federal government wouldn't certify anyone as instructors.  Good result?

Or how about this. The entire Federal judiciary only has authority as the Executive Branch carries out their decisions.  If you don't follow a Federal Court's order, all sorts of things can happen to you, but the Court isn't going to do it.  To give an example, if a state ignores Obergefell, and there were some states that seriously gave some thought to that, it isn't as if Anthony Kennedy and Ruth Ginsberg were going to ride into town at the head of the Federal Court Cossacks.  Nope, they have none.

 Justice Anthony Kennedy enforcing one of his more unpopular decisions on the states. . . nah. . .probably not.

So if a President decided that Obergefell was a judicial coup, which it basically was, and he cared not to enforce it, is it okay with everyone that he just says "nah. . . .don't bother with it"?  If that example sounds absurd it has in fact occurred. That's exactly what Andrew Jackson did with the Supreme Court's finding that removing the Cherokees was illegal.  He called their bluff and it happened anyway.

 Cherokee chief during the removal period. . . but  you're okay with that as long as it means the government ignore weed, right?

To a very large extent law works in a democratic society as the Executive Branch has sworn it will enforce it, like it or not. That's its obligation.  When it ceases doing that, and decides to pick and choose what law it will enforce, it's acting illegally.  More than that, it's acting dictatorially. And oddly, in fashion that encourages contempt for the law, and a type of low grade anarchy.

Don't like the law? Argue it be changed.