Perhaps I misunderstand your point, but members of those groups don't have a real say in what the others do. They don't coordinate in a coherent policy. In contrast, my US culture does have a formal political system, where I do have a say in its decisionmaking. So we share some responsibility for its violence.

That decisionmaking power is less than in a real democracy (a top-down republic with some democratic forms), but more than a dictatorship. So there's some shared responsibility which does not exist in your examples of other groups.