Tim is technically right. 1.8 bans actively instigating or supporting unjust wars. Inaction does not infringe on this, and so it does not obligate us to get involved in every unjust war, let alone in every war to verify if it is just. Shooting down any missile not approved by the World Union has little to do with 1.8.
More importantly, you mentioned that JAMSNET wouldn't automatically shoot down cruise missiles. Because their flight time is measured in minutes, it would be unfeasible to react to them. Thus, we return to the argument that cruise missiles as a WMD first strike delivery system have impunity.
Not to mention that naval-launched cruise missiles, if not specifically authorized, will be difficult to identify nation of origin in the short time they are in flight. This is especially true with submarines. We could ban Manchurian cruise missiles, but we'd not be able to detect the nationality of a Manchurian cruise missile fired from a submarine in the Pacific, and so our ban would not be worth much.