This looks like a pretty nice solution to the problem.
A better rule would reference the governmental monopoly on violence. Threatening to nuke a city is extreme violence, but there are about a dozen world leaders who can actually do that without violating their nation's law. On a smaller scale, various law enforcement agencies are authorized to commit certain pre-approved acts of violence. If the police want to threaten the use of a sniper in order to encourage surrender, that should be fine if the use would hold up in a courtroom.