Weed makes people stupid (by brain damage?). American people should stop smoking weed, because it supports cartel violence in Mexico by giving them money, and other parts of Latin America. This causes people living in these areas to feel fear often. Thus, smoking weed is morally bad.

Cartel wars causes people not in the cartels to die, and that is bad. Powerful cartels also support sex-trafficking of children, and that is bad. For Americans, smoking weed is usually used to relax after they get tired after working, a somewhat trivial purpose. Thus, on balance, the benefits of smoking weed is small, while the costs are high. Thus, smoking weed is morally bad. Americans who smoke weed are also signalling virtues (unspecified. Perhaps “open-mindedness”), and thus, smoking weed is also selfish.

The War on Drugs is good, in that it is trying to stop Americans from smoking weed. However, it has bad side effects, in the loss of civil rights, and in the overzealous force from federal efforts to fight the War on Drugs. The responsibility for these bad side effects are 100% on the Americans who smoke weed or produce weed. It’s good that Americans who do not smoke or produce weed are paying a lot of taxes to support the American government’s War on Drugs. Rationally speaking, the usual prison sentence for smoking and producing weed is too long, but emotionally, it is fine.

In conclusion, American people who smoke weed are responsible for causing a lot of pain for people living in Latin American. Thus they are morally bad.

1. Why should cartels exist in Latin America, but not America? The governments there are weak, and that should also be responsible.  
2. A significant source of cartel profit comes from cocaine, not weed.  
3. Economically, the War on Drugs is attempting to decrease supply rather than demand of drugs. This drives up the price of hard drugs (heroin, cocaine, etc. Not weed) and only slightly decreases the quantity of them, because people who need the hard drugs are still gonna buy them even if their prices double (they have “inelastic demand”). This also makes cartels more violent, forced into an arms race with government forces.  
4. While I cannot find surveys on what motivates people to use weed, I guess that virtue signalling is not the top reason. Experimentation seems much more reasonable as the main reason for trying weed, and mood enhancement/relaxation as the main reason for continued use of weed. In any case, legalization of weed could also take away the usefulness of smoking weed for signalling the virtue of open-mindedness, since weed would lose its cool of being illegal.  
5. In the 1920s, the War on Alcohol (the Prohibition) caused great gang violence in America. Similar to weed, alcohol is used for “trivial” reasons of relaxation and mood enhancement. Presumably, the War on Alcohol could have ended with people simply stopping to drink alcohol for trivial reasons. But it never happened, and the violence ended only after legalization of alcohol.