Is the obligation to wash the hand upon waking a rule based on a specific underlying cause (Illah) or an act of pure religious obedience (Tawabbudi)?

Primary text

The apparent stance among scholars who mandate washing the hand upon waking is that it constitutes a matter of pure religious obedience (Tawabbudi) rather than being predicated solely upon the specific cause of hand contamination (Ta'lilat al-Tanjis). This interpretation supports the generality of the command, as the ruling does not necessitate declaring the hand or the water explicitly impure.