Weapons seized from members of the gang Los Zetas are displayed by police during a presentation to the press, in Mexico City. | Luis Acosta/AFP via Getty ImagesThe Supreme Court handed down a unanimous opinion on Thursday that shuts down a lawsuit brought by the nation of Mexico against US gun companies. In Smith & Wesson v. Estados Unidos Mexicanos, Mexico sued seven American gun manufacturers, claiming that their products are often sold to gun traffickers who then provide these guns to Mexican drug cartels. The Mexican government claims that up to 90 percent of guns recovered at crime scenes in Mexico come from the United States.Unfortunately for Mexico, however, a 2005 law known as the Protection of Lawful Commerce in Arms Act (PLCAA) gives American gunmakers broad immunity from lawsuits seeking to hold them liable for harms “caused by the misuse of firearms by third parties, including criminals.” PLCAA does contain some exemptions to this general rule. As Justice Elena Kagan explains in the Court’s Smith & Wesson opinion, a gunmaker can be held liable for “aiding and abetting someone else’s firearms offense.”Mexico claims that the gunmakers aided and abetted illegal sales to cartels by “supply[ing] firearms to retail dealers whom they know illegally sell to Mexican gun traffickers.” Mexico also faults the companies for allowing bulk sales of guns, which can enable illegal sales, and for practices such as designing guns that appeal to Mexican culture.One such gun, for example, features an image of the Mexican revolutionary Emiliano Zapata, along with a quote from Zapata: “It is better to die standing than to live on your knees.” But Kagan’s opinion concludes that the mere fact that US gun companies likely knew that some of their guns were being resold in the illegal market, much less that some of their guns are designed to appeal to Mexicans, is not enough to overcome PLCAA. As Kagan explains, this conclusion largely flows from the Court’s fairly recent decision in Twitter v. Taamneh (2023).Twitter concerned an attack by the terrorist group ISIS that killed 39 people at a nightclub in Istanbul, including a man with American relatives. Those relatives sued several social media companies in US court, claiming that the companies aided and abetted the Istanbul attack by allowing ISIS to post content which promotes ISIS’s ideology and that attempts to recruit people to the terrorist organization’s cause.But Twitter warned against a legal regime where “ordinary merchants could become liable for any misuse of their goods and services, no matter how attenuated their relationship with the wrongdoer.” As a general rule, someone who provides a good or service to all comers is not legally responsible if a bad actor uses their product for a wicked purpose. If Ford sells a truck to a man who intentionally uses it to run over and kill his wife, Ford normally will not be responsible for this homicide.And so Kagan concludes that it’s not enough for Mexico to show that gunmakers could have taken additional steps to prevent their products from winding up in the hands of drug cartels. Instead, “the merchant becomes liable only if, beyond providing the good on the open market, he takes steps to ‘promote’ the resulting crime and ‘make it his own.’”Of course, one thing that distinguishes Smith & Wesson from Twitter is that social media platforms are not weapons whose entire purpose is to injure people. If PLCAA did not exist, Mexico might have argued that the gun companies’ decision to make and sell an inherently dangerous product should make them liable for the consequences of selling such a product.But, of course, PLCAA does exist. As Kagan writes, “Congress enacted the statute to halt a flurry of lawsuits attempting to make gun manufacturers pay for the downstream harms resulting from misuse of their products.” That may be a bad choice on Congress’s part. But, absent a constitutional violation, it is not the Court’s job to second-guess Congress’s decision to set national policy.