I thought Straight Talk stopped selling AT&T SIMs a few months back because they started getting too expensive at wholesale. And AT&T has since dumped part of their share in Straight Talk's parent, America Movil. So Straight Talk is primarily Sprint + Verizon + T-Mobile now.
I was not aware that the ATT Sims were basically not available at all anymore on either StraightTalk or Net10 for BYOD unlimited plans. As recently as a week ago I looked on Net10's site and they still had them. Either way, the point still stands, there's many other MVNO's worth looking into that are cheaper and offer better customer service than ATT's GoPhone.
Well, yes. And you can get Straight Talk on AT&T by getting a GSM Straight Talk smartphone, as it turns out. But it looks like Tracfone is doing everything in their power to prevent you from buying an AT&T SIM kit from them.