Your mistaken. U.S. citizenship is, quite rightly, one of the most precious possessions we have. And U.S. law makes it clear that citizenship can be lost only if the citizen himself gives it up. It cannot be done for you, and can only be done by you if you're an adult. So Obama would have to give it up when he was 18, not reclaim it. And he can only give it up through deliberate actions. The Supreme Court has ruled that someone cannot lose their citizenship by accident. The act must be deliberate and done with the full knowledge that the person is giving up their citizenship as a result.
Even if this is so (and I am not conceding the point), applying for, receiving, and using an Indonesian psssport were actions taken by Obama himself.