I might be able to shed some light on this...
I travel to the US frequently. I've been using factory unlocked iPhones since 2008 in Canada (Rogers) and in the US (GoPhone). I use to buy blocks of data and use the APN changer and it was all good.
On my latest trip at the start of May, when I tried to purchase more data - my data connection on my iPhone stopped working. When I called CS, they explained the deal with iPhone on GoPhone to me. I told them I would use another phone and they turned my data back on. It actually didn't work until I put my sim in a friend's phone (a Nokia E5 or something equally as crappy) and turned it on. After that, the data started working again in my iPhone after I put my sim back in. At some point, my data package expired and the data stopped working again.
The agent explained that all 4G devices are not allowed to use data on prepaid GoPhone accounts. Even if you buy the $50 plan and add a data package - it will still not work if ATT knows you have a 4G phone.
I think I have two options, carry a second phone (non 4G) and keep swapping the sim card between the phones when I need to buy data or switch to T-Mobile.