The dominant political unit had a legal right to demand services of the general population. Guelich remarks: “Commandeer [for compel] represents an old custom stemming from Persian origins that required one to perform or to grant services according to another’s wish. The Roman soldiers apparently maintained the detested practice in using Jewish civilians for their purposes. For example the Romans commandeered Simon to bear Jesus’ cross (Mark 15:21). Mile was a Roman measurement of about 1000 paces, just less than a mile. This request was legally binding.”
Although being commandeered in such a way was legal, the law was imposed by the conqueror, and it would have been natural to begrudge the labor thus imposed. Jesus requires the person commandeered to give up his desire for retaliation, to perform the required service, and to offer an additional measure of service. What is freely given cannot be commandeered. What is given away cannot provoke retaliation.
Book of Mormon Context: Because the Persian practice (and its Roman sequel) came after Lehi left Jerusalem, the Nephites probably had no experience with this type of law. Even though this particular example would have had no cultural referent in the New World, the principle of foregoing retaliation was still important.