Which is walking a fine line between madness and genius. A very fine line. :P
A good DM is a little bit genius, a great DM is a little bit of both. :P
But this tension should exist on its own from the world's and characters' perspective, and not be enforced ingame by literally translating modern concepts into the game.
I think I lost you. My contention is that tension in the way the character's interact with the environment and attitudes of NPCs, cultures, and societies (essentially; social norms, religion, faith, law, order, and morality) is a good thing, and makes for a better game, if it is done well. Included in all of these things listed above you can successfully integrate modern views, and have tension and fun. I am not sure what you mean by the "enforced" comment.
Simplistic Example for Illustration: Modern Concept - Universal Sufferage.
A city state in a POL campaign may allow all of the members living in its borders to vote, viewing that they all have a stake in their mutual safety. A neighboring city state may refuse to allow women to vote, hold office or status. Yet a third city state may be run by a powerful female warlord descended from a matriarchal tradition, and not allow men the right to vote. You could substitute the male/female dichotomy for any races or combinations with any sociocultural background.
The tension for characters travelling between the three becomes obvious, and can be a lot of fun. This kind of game isn't necessarily for everyone, and it is not so much for the DM to enforce one view over the other, as to create the dynamic environment for the characters to interact in.
Finally, a true god of justice would have to be unaligned (or true neutral), because anything else would be prejudiced.
You make a good argument, a long time ago I once played a fanatic druid that viewed true neutrality as being the ultimate badge of justice, and he viewed himself as being the righteous justicar of nature.
But gods are frequently egotistical, far and aloof, which means their motives, goals and decisions are often beyond understanding. I think this is one of the reasons Wizards decided to distance them in the basic campaign setting.
I think the most important thing about a DM is that their reasoning be sound, and not arbitrary.