There is a distinction between what is a sin and what is a crime. A sin is a violation of a law of God, while a crime is a violation of a public law. An act can be both, but many times it is not.
Alma sought King Mosiah's help in dealing with sinful beliefs and practices being spread among the people: "And he said unto the king: Behold, here are many whom we have brought before thee, who are accused of their brethren; yea, and they have been taken in divers iniquities. And they do not repent of their iniquities; therefore we have brought them before thee, that thou mayest judge them according to their crimes" (Mosiah 26:11). Mosiah, however, could not involve the government in Church matters. Even though the beliefs of some people were evil, Nephite law allowed them to believe as they chose (see Alma 30:7-11). In this sense, the people had committed no "crime." As the head of the Church it was Alma's responsibility to judge such people, so he turned to the Lord for instructions (Mosiah 26:15-32). [Book of Mormon Student Manual for Religion 121 and 122, p. 68]