Prove it
The Expatriation Act of 1954. Loss of citizenship, whether by birth or by naturalization, has to be a deliberate act performed by the citizen themselves. It cannot be done for them, as by a parent or guardian. For Obama to have lost his natural born citzenship he would have had to have performed one of the several expatriating acts outlined in §349(a) of the INS code. The two which are most relevent are:
1) Obtaining naturalization in a foreign state upon the citizen's own application or upon an application filed by a duly authorized agent, after having attained the age of eighteen years. In other words Obama would have had to seek citizenship status on his own when he was 18. Or
2) Taking an oath or making an affirmation or other formal declaration of allegiance to a foreign state or a political subdivision thereof after having attained the age of eighteen years. In other words Obama would have had to formally renounced his U.S. citzenship.
The acts have to be deliberate. They must be done with the full knowledge that Obama intended to give up his citizenship. And the burden of proof, according to the Supreme Court, lies with those claiming he did intend to do so.
Indonesian law is irrelevant in this. Regardless of what it says on the matter it does not supercede U.S. law. Obama's mother could no more give up citizenship on Obama's behalf than he could give up her's. If he was born here then he is a U.S. citizen, and has been from the day of birth to this day. Unless you can show otherwise. If everyone is correct and Indonesia does not allow dual citizenship, then the most likely conclusion is that Obama's Indonesian citizenship, if it existed, was fraudulent and void.