It wasn't an oath to God - it was a pledge to defend Islam.
An odd thing for a head of state to swear to, in my mind.
We don't make our Presidents swear to uphold Christianity. Not sure of the words to our oath, but I imagine it goes something along the lines of swearing to uphold the Constitution, so help him God. Something like that, right?
Very different than what Karzai swore to, and not only that, it was a distinctly separate oath from the oath to office. I was looking on google from the text of the oath of Alligience to Islam but no one carried it to that detail.