As a police tool, the current deployment of tear gas reinforces the effect that made gases so powerful in World War I: fear. With reports showing tear gas being used on peaceful protesters, that fear is itself a deterrent by law enforcement on public demonstrations. Given legitimacy by the CWS in the interwar period, tear gas provides police with a chemical weapon that is no longer permitted in war.

https://daily.jstor.org/why-do-police-use-tear-gas-when-it-was-banned-in-war/