Have you never heard of common courtesy? I'm not by law required to do a lot of things I do but I do them because they make the world that little bit better for everyone, same goes for things I avoid doing.
"Common courtesy" is a very vague term and changes from place to place
And thanks to all my know-better self-righteous downvoters. I didn't know questions were inappropriate.
If you can't rigorously define something, you certainly still can work with it, pursue it, use it, regulate it, etc. Consider: art, love, health, happiness, pregnancy, ...