After a divorce, why do parents make up negative stories about each other?

Can the mother destroy a father's credibility by cutting them off from their aunts and uncles, cousins, grandparents and even the father himself? How is it that is permitted? And that even if family courts petition the mother to do otherwise, that she can still ignore the legal ramifications and still continue to destroy a father image altogether without having the courage to tell the children the truth even on her death bed?