Look, the fact that a given group of people can do X in a given situation is not a solution to other people doing Y in another situation. Just because you or I would not litter anyway, it does not mean no-littering signs are unnecessary nor that there is not a difficulty with enforcing them. It is different people, probably in different situations, circumstances, even with different litter.

This is a common problem IMHO and generally I think the best conceptual model is to think that the good part and bad parts of human nature don't cancel each other out, they exist side by side. So for example power-hunger or aggression and charitability are both being parts of human nature, but not canceling each other out, but operating side by side.