From OM15:
The Watcher may add to the doom pool directly by spending effect dice from a villain’s action, rather than inflicting stress or creating a complication. This represents the villain causing general chaos and mayhem, threatening innocent bystanders, or grandstanding.
I can see general chaos coming from a villain acting against a hero (villain attacks hero, smashes hero into terrain feature, setting things on fire in the process), but if a villain wants to do an action that doesn't target a specific hero, what happens? Heroes can roll against the doom pool, but villains rolling against the doom pool makes no sense.