Engadget was given another reason upon investigating. AT&T claims credit card fraud in the city has halted online sales. That one makes little sense to me as AT&T would have no reason to suspect a customer of credit card fraud if they are an existing customer that pays with a credit card. When sites suspect credit card fraud, there are additional precautionary measures that can be taken, like ensuring the device is sent only to the billing address or other addresses that are registered with the card. You don't simply halt sales, and you certainly don't halt sales of one device.
Apparently you can still buy an iPhone at a physical retail location. That makes some sense even if the overloaded network is the culprit. In a city that probably has hundreds of thousands of iPhones, selling a few hundred or even a few thousand more of a known quantity remaining in inventory isn't going to make a big difference, whereas having a potentially unlimited number of sales online could. As Christmas ends and people have gift cards and cash to spend, this could be a big week for cell phone buyers looking for that phone that wasn't under the tree.