This is problematic because one of the underpinnings of western liberalism is the notion of Rule of Law: that, at the end of the day, we are all bound to the same rules.
Right, and one of those rules is "heads of state are exempt from all rules". Otherwise Obama would be getting sued for ordering drone strikes and we can't have that.
They are governed by the rules determined by whoever decides what the hell exactly "the rules-based international order" is — or rather they're not, much as some like that to be the case.