Both of your paragraphs can illustrate the point of what the dilemma is here. In your first paragraph you illustrate natural law, and in the second you illustrate Singapore law. In this case there is a conflict between natural law and US law. It could be argued that the archdiocese didn’t adhere to the “tough” US laws in the regard.
This is a fair point. The problem is that the archdiocese has to provide the burden of proof that she should have known. If the archdiocese can prove that she understood that what she did was a violation of her contract, then the archdiocese would be in the right here.
Was she provided a copy of the Catechism of the Catholic Church and told if she had questions about the morality clause to obtain clarification? If so, then I agree with the archdiocese. If in her contract it stipulates that she will be provided a copy or access to the CCC, I think the archdiocese has a solid case. If not, it seems to be undue burden on someone who may see Catholicism as just another Christian religion that believes in Jesus but has more rituals (for example) than other Christian faiths.