Because we're not talking about human beings. We're talking about huge, huge collectives of humans that are impossible to trust or predict. Think of states like you do of corproations: You wouldn't trust a corporation to do "the right thing" or "act nice", wouldn't you? Now image the corporation has nukes and tanks and lives next to your back door. We have a saying: "Trust is nice, surveillance is better" (which is used in different context but applies very nicely for this situation).