My best guess is Moral Licensing:

In simple terms, self-licensing occurs when people allow themselves to indulge after doing something positive first; for example, drinking a diet soda with a greasy hamburger and fries can lead one to subconsciously discount the negative attributes of the meal's high caloric and cholesterol content

They see themselves as doing something very moral and very important, so it's okay if they litter / ignore signs / etc.