Could AT&T be dumb enough to require this in hopes of avoiding churn/switching? I have to believe it's easier to minimize the number of models.
Not much new info in this story -
http://www.pcmag.com/article2/0,2817,2491013,00.asp. But even the above reason doesn't seem to make sense. AT&T phones will work fine elsewhere, but other phones won't be able to use AT&T's Band 30 network.