He did not have to. The Indonesian oath of citizenship is not binding under US law. To renounce citizenship, you have to state before a consular officer that you intend to renounce your citizenship. Or undertake specific acts that show intent. Did you not read about the Marc Rich case (remember him?) referenced in my post concerning the Spanish oath of citizenship? It's provision renouncing prior citizenship was not considered binding by US law, because of Marc Rich's lack of intent to surrender his US Citizenship.
No law of any other nation can strip an American of his citizenship. Indonesian law only determines Indonesian citizneship. Only US law determines who is an American.
Why is there such a law on our books?
You cannot answer that, I can and it is simple, our government recognizes that a minor child’s US citizenship can be removed by the actions of a parent, that’s why we have a means by which someone that reaches an age where they can express their desire to be an American citizen can reaffirm that desire for the record and that they have a choice to do so or not by their actions.