This sort of thing would almost certainly happen in an anarchist society, but this is not really the kind of “law” I’m talking about. You’re talking about different groups of people who mutually agree to resolve their conflicts by a certain method (e.g., Sharia courts) while leaving non-members out of it. This “agreed-to law” is different from the more fundamental law that tells us, among other things, who is bound by the Sharia rules in the first place. By saying “Only members of the Sharia court have to wear hijab, and non-members don’t,” you are making an argument about this fundamental law, which (supposedly) cannot itself be opted out of.
The Cowan/Sutter arguments apply to the extent that people interact with each other. To that extent they have to agree on a common law between them. For example, the local Christian and Jewish communities in ancient times might each have different rules regulating marriage and family, and nobody cares because there is no intermarriage. However, if there is commercial activity between the Jews and the Christians, then there will be pressure to settle on a common law, at least for the purposes of commerce. The more globalized people’s activities become, the more there will be a tendency towards a common law, and we’d better make sure it’s not “Do whatever the government says.”