Wendy McElroy writes: Ignorance of the law is an excuse…if you are a cop. American police no longer need to know what the law says or to enforce it correctly. They can implement a non-existent law with impunity even if it results in the apparent violation of constitutional rights. This may have been apparent to many as a police practice but now it is now officially the law of the land. On December 15, the Supreme Court of the United States (SCOTUS) ruled on Heien v. North Carolina.