You have to remember what the carriers were like in 2006/2007 timeframe. Cingular/AT&T was the open carrier at the time. You would use any GSM phone. More importantly, they allowed you to use any feature on your phone. Conceding to Apple control of the apps on your phone was a very radical idea at the time. Typically carriers dictated what application would be on the phone. On Verizon, you had to use their phones (there was some lipservice around this time about allowing any fully compatible phone on its network). In addition, included features such as bluetooth PC syncing, which had zero impact on the network, was disabled on my Nokia candybar. It was simply too much of a leap for Verizon to allow a phone as open as the iPhone (2007 terms, remember) on its network.

T-Mobile USA was just as open, perhaps a bit more because you could use European GSM tri-band phones.

It would appear that they didn't want to develop non-GSM headset, and the only two choices for GSM in US is AT&T and T-Mobile. T-Mobile is even slower to roll out network upgrades than AT&T.

Apple developed and maintained X86 OS X for years when Macs were PowerPC-only.

To me it seems likely that they have CDMA iPhones "just in case" as well.