If you're in a no passing zone, it doesn't matter who the driver of the car ahead of you is. You aren't allowed to pass. If you get pulled over, you can't say the no passing zone doesn't apply to drivers with out of state plates: it applies to you. Likewise, the government is not allowed to do certain things. Period. It doesn't matter who those things are done to. The Bill of Rights "applies" to the government.
The "loaded gun in the wrong hands" is terrorists claiming protection under our bill of rights. They can't get under our umbrella. It doesn't matter if they're persons. They're not "of the United States". Our umbrella is not for them. They have no IVth Amendment rights.