I think you must put the concept of extraterritoriality in the context of the times. Imperialist Europeans had judged the rest of the world based on the idea of "social darwinism". They had come to believe that Charles Darwin's theory on evolution and the survival of the fittest also explained the world politically, economically, socially & culturally. Countries with superior technology could dominate weaker countries, therefore it could be rationalized they were the superior race, culture etc. and were suppose to evolve and the other, weaker countries were seen as inferior and would eventually be absorbed by the dominate culture or disappear. With this attitude came the belief that their laws, government, religion and basically all aspects of their society were superior. It helps explain extraterritoriality along with the missionaries purpose to "christianize". Of course we look back now and realize its nothing more than racism at its best.