Simply to force his hand. And in a nonchalant way, he did. The military agreed with the soldier, obamatard has a shroud of reasonable doubt around his eligibility.
Just because he was sworn in doesn't mean it was legal. Just because everyone on earth but me says the sky is hot pink doesn't mean it is...
Sorry SE, but having his orders canceled does not mean that the military agreed. He volunteered to go. Then, before going, he withdrew the volunteering. Since he was not active duty and since his unit wasn't deploying (but he had volunteered as an individual), he could do that right up until the time he actually embarked.
Per regs, once he withdrew his volunteering, his orders were cancelled. Since no reason is necessary for any such withdrawing of volunteering, the reasons he stated for such withdrawal don't enter into it.