I think the oni thing can be thought of as a "system".
The Kami are being that came to earth, probably attracted by the prayers of humanity, whatever they take from those prayers it is just a "system" where energy is transmuted, as powerful as they are they aren't immune to the changes of life or the "authority" of the "system".
The oni are a "system" by itself, one that is exclusively human, and probably has to do with humanity's karma, most probably with only the Japanese karma, at least in the form it takes in this story, as so it is a "system" that is outside of the Kamis influence to such a level that they are forbidden to interfere in any sort of manner.
Only now I noticed... since chapter 21 on the cover Ozuno has a Uber pack... and now he is delivering the treasures to Azuma... that is an expensive Uber XDDDDDD.