For the same reason a 3G iPad is not an iPhone. It doesn't include Phone.app and doesn't require a phone contract.
People who buy iPod Touches use them as pocket computers. It isn't surprising they'd like to have network access everywhere instead of just wifi. That's already a reality with the iPad, and it makes sense that they would make other devices work with those same pay as you go data plans.
So this is just iPhone without the phone.app. Do you know how much iPhone costs without AT&T?