Though marijuana remains illegal under federal law, states cannot be forced to ban it under their own state laws, just as states are not required to ban other activities criminalized by Congress. The Supreme Court has ruled that the federal government cannot compel states to enforce federal law, and it is even more obvious that fellow states cannot do so. It is ironic that this lawsuit was brought by two state governments that, in other contexts, have forcefully advocated state autonomy and tighter enforcement of constitutional limits on federal power.
In a dissent joined by Justice Alito, Justice Clarence Thomas argued that the Court should have taken the case, because it has no authority to refuse to consider a lawsuit that is within its mandatory jurisdiction (in this case, a dispute between two states). However, as Thomas recognizes, the Court has several times refused to consider mandatory jurisdiction cases in the past (presumably because the justices believed the case was too weak to merit a hearing).
I think it might have been better if the majority justices had briefly summarized their reasons as to why the Nebraska-Oklahoma lawsuit lacks merit, instead of rejecting it without any explanation. But, on balance, I think it was appropriate for them to refuse to consider it. Just as lower courts have various procedural devices for dismissing meritless lawsuits without a full hearing, so the same should apply to the Supreme Court when it considers cases within its mandatory jurisdiction. However, I am not an expert on the law of mandatory jurisdiction, and therefore recognize that there might be an angle here that I am overlooking.
Whatever might be said of the procedural aspect of the Court’s ruling, substantively the termination of this lawsuit is an important victory for constitutional federalism.
UPDATE: I have made a few minor revisions to this post. Because they were added within minutes of initial posting, I will not go over them in detail here.