Considering Straight Talk runs on AT&T network...it is technically a Official Carrier for iPhone....so apple assumes and I bet the APN that gets "Automatically" selected is the AT&T APN, As for T-Mobile USA it is not a Offical Carrier of the iPhone, therefore Apple does not add its Carrier.Plist bundle within the iPhone's Carrier Bundles....that is why most of you probably see the CDN available to you rather than when you are on any AT&T MVNO(Straight Talk, H2O Wireless, Red Rocket, AirCel etc).
***This info is purely a prediction as to why CDN isn't available for users on AT&T MVNO's....I have gone through my carrier bundles and don't see T-Mobile USA...I hav a Factory Unlocked 4S...I bought tetherMe Years back when it was super cheapso its been nothing but helpful.
I can confirm this. I just unlocked a 3GS, put in an AT&T SIM card, CDN doesn't show up (along with Carrier selection). With the Straight Talk SIM card, it was also hidden. When I stuck in a non-US SIM card, both CDN and Carrier selection showed up.
Thanks for the tip!