One of the problems with a system of local governance based on family loyalty is that, if someone offends your family, you have a personal responsibility to get revenge. Look where that gets Alan and James Stewart in Kidnapped: the two of them shoot their mouths off all over Scotland, talking about how much they resent Colin Roy for taking taxes from their clan chief's estate. Then once Colin Roy is shot, they of course become the prime suspects. And the cycle of violence continues. Davie and Alan can't rely on the impartiality of a Highland court if they go on trial for Colin Roy's murder. The trial would be held in Inveraray, stronghold of the Campbell family, where they would be certain to be convicted out of vengeance for Colin Roy's death.
The clan system is incompatible with the whole idea of a jury by one's peers, because a courtroom full of representatives from one family cannot be unbiased.
Davie's choice not to prosecute Ebenezer publicly for kidnapping him demonstrates his concern for his family name over his need for revenge.