Which authority dictates/regulates that in computer science a journal paper cannot be presented in a conference?
As is, a journal paper cannot be presented in a conference in many fields, not only in computer science, whether the conferences are counted as publications or not. Of course, you can always present an improved version of a previous work, if the improvement is significant.


  It seem that this policy in CS emerged from the community (bottom-up) (e.g. see this), sort of hand shaking or bowing in Japan - and therefore self regulated by people.


The entire academic world is somehow self-regulated by people, and most people agree on the fact that trying to resell the same paper twice is unethical because it can lead to an artificial inflation of the number of publications, which is one of the many indicators employed to hire people in academia. 

And even if you don't find that unethical, you don't want to read the same publication again and again, do you?