Why does everyone think he didn't call to cancel the AMEX?

I expect there will be a chargeback, it the card on file is the lost one and has been canceled, but since the OP is a "known entity" to Starwood as a Platinum, I'd wager a quality FDM would take the chance that it will be rectified.

Or the card stolen isn't the one on file? Corporate vs. personal, perhaps?