I don't think AT&T has any control over their inventory - isn't it interesting that people are still walking in off the street at Apple stores and are able to purchase any model??
Not sure where you are getting your data, but you obviously haven't looked at Apple's site. They are sold out of iPhones in many Apples stores and black 16 GB models are sold out in majority of Apple stores. They have been for days.
I think AT&T had more to lose from selling the iPhone online versues in-store activation. Whether you buy the iPhone at a retail store or an online site, Apple is getting paid. But if you buy it online and jailbreak it to use on a different cellular network, AT&T loses money. According to the NY Times, 2/3s of all original iPhone sold were never activated to their proper cell service. (people unlocked it to use on Cingular, etc)
Even if the iPhone was available online, you would not be able to get one. They would have sold out at a much faster rate because only people patient enough to wait in line are buying them in stores. If anyone could just order one from the site, they would. And it would be back-ordered for weeks. Too much demand, not enough supply. The only fault of Apple is making a product that people adore so much.
Apple did make some mistakes, mainly with the software update. (having current iPhone users upgrade on the same day 3G users needed to activate their phone) I think if the software update was available a few days prior to the launch, things would have gone much more smoothly.
But, the original posters argument of not allowing a internet-enabled device to be purchased over the internet is a weak one. The Nintendo Wii (which is an internet-enabled device and a revolutionary gaming console) was sold in stores and online and was VERY hard to purchase for almost a year due to the high demand.