The best way to make this case is to say that not all offenders are created equal. This is true even of offenders who are charged with committing the exact same crime. Because not all offenders are equal, it can be fair to treat different offenders differently.
In a system of informal justice, the system is trying more to ensure that justice is done and that the best thing is done for the community. This is in contrast to a more formal system that tries to ensure that the same rules are followed in every instance. An informal system can arguably be better for a community as a whole. For example, let us imagine that two people both commit burglaries. One is a person who has been convicted of a number of previous crimes. He has no job and no real support system in the community. The other is a person who has no previous criminal record. He burglarized a store on a dare. He is set to go off to college in a few months if he does not go to jail.
It is at least arguably better for the community if the two offenders are treated differently. Putting the second offender through an informal system is more likely to help the community. If this person can still go to college, he is more likely to become a productive member of the community than if he is jailed.
Because crimes are committed by different people in very different circumstances, it can be fair and appropriate to treat some offenders informally and others formally.