Ignorance of the Law of the Land

In the law of the land, if you break the law ignorance of the law is no excuse.

Moral law is far more useful and important than the law of the land so it seems hypocritical how Christians say that ignorance of the moral law is an excuse if you really don't know any better.

The law of the land protects public morality. For example, it stops a person who riots just for fun. Ignorance of the law then means ignorance of public morality. Why would we say it is no excuse to plead ignorance of the law of the land and excuse those who do not know right from wrong?