It is not against code to add a shut off on the hot water side and it makes it easier to service the equipment. The TPR valve is what protects the equipment. If you don't have one and all of the hot water fixtures are in the off position, it is no different than having a valve installed on the hot side and in the closed position.
I understand your concern and appreciate your passion but you are incorrect.
I think you need to specify the code that prohibits this, otherwise you are misleading people with false information.
A shutoff valve is required on the cold water side per the IPC and IRC. In the IPC it can be found in 503.1 and in the IRC it is in P2903.9.2
I have supplied you with direct references, now I ask the same from you.