AT&T is not the only carrier, having this issue. If you knew anything about deductive logic, or isolation troubleshooting, you would have chosen your words differently. Since all 4 major American Carriers are having the issue, you have to look at the common denominator: iPhone.
Now, since the 3GS, 4, 4s, and the 5 are having the problem, you then have to look at the common factor of those 4 devices: iOS 6.1.3. All of them can run the latest software. iOS 6.1.4 only corrects software comparability, with the speaker on the iPhone 5.
The software, that all the major carriers use, for their billing system is identical, made by the same company. So to say it is any of the carriers' fault, would lead one to believe that all devices offered by a said carrier would have the same issue. However, my Windows Phone has no issues with charging cellular data, when I am connected to Wi-Fi.
The issue stems from iOS 6.0.1. There was a switch titled, "Wi-Fi + Cellular", which gave the user the capability to disable cellular or enable it, when the phone was screen locked and asleep. That switch is no longer available, and the wi-fi chip is disabled during sleep mode, to reduce battery drain.
Disable your location services, as well. iCloud, Siri, and many factors of the iOS are running your location. You van configure the settings, under Settings>Privacy>Location Services.
iOS 7 will be here soon. I have got to preview the beta, and I am hopeful, that the issue should be resolved. Until that time, stop pointing fingers, at your carrier, and start Google searching the issue. Apple has contracts with some of the carriers, that offer the iPhone.
Next time you wish to make accusations, have evidence to back your claim. Otherwise, your post will be shot down fast