The activists behind the Arms Trade Treaty (ATT) have never been shy about blaming the U.S. and other democracies far more than they blame dictatorships for the evils of the world. Their latest campaign shows they’re at it again.

The campaign centers on the conflict in Yemen, a war-torn nation on the Arabian Peninsula. In the late 2000s, a group known as Ansar Allah (more commonly called the Houthis) launched an insurgency against the Yemeni government. In 2014, Houthi fighters captured the capital city of Sana’a, and in early 2015, they formed a revolutionary committee to govern the nation.

But the Yemeni president, Abd Rabbuh Mansur Hadi, escaped and from Aden, the former capital city, described the Houthi takeover as illegitimate. Since then, a war has pitted the Hadi government against the Houthi Revolutionary Committee, which controls the western portion of the nation bordering the Red Sea. The Hadi government is backed by extensive Saudi involvement, and the picture is further complicated by occasional U.S. airstrikes against an ISIS and al-Qaeda presence in the region.

The Saudi government, in turn, buys arms from many nations, including the U.S., Canada, and Britain. This is where the Arms Trade Treaty comes in. For the past month, activists have claimed, increasingly loudly, that these arms sales violate the Arms Trade Treaty because the Saudis have used the Western weapons to commit human rights violations. The activists also point to Saudi Arabia’s use of public executions as another reason to halt arms sales to the kingdom, and to condemn it in general. There is more than a whiff of support for the Obama administration’s outreach to Iran in these stories.

That may be because Saudi Arabia and the Western nations are not the only foreign nations involved in the Yemeni conflict. Though the activists deny this on occasion, U.N. experts have found that Iran has been arming the Houthis since at least 2009. The seizure of an Iranian boat carrying missiles and other arms in late Sept. 2015 further confirmed suspicions of Iranian involvement, which violates a U.N. Security Council resolution.

Without Saudi involvement, and without Western arms, it is likely that the Houthi rebels would win, and that Iranian influence in the region would increase. The results of that could easily be worse—both for human rights and the stability of the entire Middle East—than the Saudi campaign. You don’t need to believe that the Saudis are great, or that their campaign in Yemen is being fought with scrupulous attention to human rights, to believe that a Saudi victory is still, on balance, preferable.

That, of course, is a judgment call. But this kind of judgment call is exactly what the Arms Trade Treaty forbids. You can’t legislate judgment—you can only ban the opportunity to exercise it.

The Arms Trade Treaty sets out certain standards for arms exports, and it requires exporters to judge their exports one at a time, in the context of the virtues and vices of a particular importer. It’s not designed to allow good signatories to help the bad fight the worse, especially if the good know that the bad are bad.

Indeed, it’s not even designed to allow signatories to arm anyone who is less than good. The structure of the Arms Trade Treaty—like the ideology behind it—inevitably leads its proponents to focus on the evils of the West’s arms transfers, not Iran’s arms smuggling or the benefits of preventing Iran from winning. And inevitably, academics are now beginning to beat the drum for the claim that the Arms Trade Treaty isn’t nearly strong enough.

But if they’re so outraged about arms sales, why don’t they protest Iran’s arms smuggling, or its violation of the U.N. Security Council resolution, with equal vigor?

It’s fitting that the activism, for now, is focusing on Britain, because Britain was more responsible than any other nation for launching the Arms Trade Treaty. Predictably, it’s now being pilloried by the very activists it empowered and encouraged. But this is too serious a matter to smirk about.

Treaty proponents like to claim that they stand for morality, but they are no more moral than the non-interventionists against whom Christian theologian Reinhold Niebuhr struggled in the 1930s. They claim that it’s imprudent for the West to transfer arms. What is really imprudent is to believe that this is a world without any shades of gray, and that the right thing to do if we can’t help someone who’s morally pure is to sit back and allow regimes like Iran to win.

Helping the bad against the worse is a moral act. It is better than letting the worse win. It is not something to be done thoughtlessly; we need to be humble about our ability to discern bad from worse, and we should pressure the bad to be better—but it is also not something to be ruled out by force of law or categorical treaty provision.

The Arms Trade Treaty is a bad treaty for lots of reasons—but as the agitation over Yemen shows, one of its worst features is that it’s arrogant in its one-sided refusal to recognize the evils of inaction.