English settlers brought the common law to the United States because it had existed in Britain long before the settlers started moving to the United States. The common law dealt with everyday occurrences of law breaking and was about precedent. If a judge made a decision on something in a certain way, then future judges would have to make the same decision for consistency purposes.
Slave owners also feared that by placing enslaved persons in the army, there would be an expectation that they would be freed based on their service. Therefore he specifically prohibited bringing blacks into the army's ranks initially.
