OP actually wrote that restaurant.com is offering a credit, not a refund....

And no, my intent would not be to "make a point" with the non-hassle, 30 second process for initiating a dispute with Amex. I said in my original post that I don't use them that frequently, so I wouldn't want to have a credit hanging around out there. I fully understand that using any variety of coupon code, OP's out of pocket cost might have been as little as $15. Still, whether it's the restaurant itself or restaurant.com, the point is that you've changed the terms after I paid, which is grounds for a chargeback.