A law is successful primarily because the behavior it prescribes has attained the status of custom.
Evaluate
Since this question stem is asking what sort of Comparison the argument makes, we should primarily read with that goal.
The structure of "Just as X , so Y" implies a comparison between X and Y.
We're comparing Lucky Charms cereal to Ben & Jerry's ice cream.
In this argument we just read, it said,
We know we're comparing manners and societal laws. What is similar about them?
They both get their authority mainly through our habit of obeying them. Contrary behavior becomes unthinkable because abiding behavior becomes customary, not because people are afraid of the penalties that await were they to violate a rule of manners or law.