There is a strong incentive for me to use a GSM phone over a CDMA phone because the former works in the majority of the rest of the world. So if all the GSM pho

There is a strong incentive for me to use a GSM phone over a CDMA phone because the former works in the majority of the rest of the world. So if all the GSM phones sold in the States were sold by AT&T, it is likely that I would be getting a worse deal then I would otherwise. It's just one way in which AT&T and Verizon do not sell directly substitutable goods.