Unfortunately that poses significant justice issues. If every city-state can impose their own form of justice, you would have an exponential increase of injustices where a legal act in one city-state could be illegal in another. While the current system may be imperfect in that regard, there are more safeguards in the form of federal level standardization that limit the inter-state legal issues. Yes, a common set of rights could be established by treaty, but it would be less likely to be universally applied or accepted when silos have the power to make their own rules and change them on a whim.