Are you sure? Could it be that AT&T thinks that AT&T->Apple, and only AT&T->Apple, can and should decide what software will run on "their" phones?
If you compare the attitudes of cellphone manufacturers with cellular network carriers over the last decade, the manufacturers have almost always wanted open solutions, while the carriers have almost always tried to keep things closed. Apple is, in this case, acting much more like a carrier than a manufacturer, and I think that this might say less about them, and more about their relationship with Cingular/AT&T.
If you compare the attitudes of cellphone manufacturers with cellular network carriers over the last decade, the manufacturers have almost always wanted open solutions, while the carriers have almost always tried to keep things closed. Apple is, in this case, acting much more like a carrier than a manufacturer, and I think that this might say less about them, and more about their relationship with Cingular/AT&T.