According to report from ModMyi, AT&T was cracking down on tethering. It seems the first round appeared to users on AT&T unlimited data plans that used more than 10GB of data in March and second appeared users using more than 5GB.
It was also said that many users have reported calling AT&T and were asked if they were using Netflix, Pandora, etc. Some have been told that using those services is the definition of tethering. On the other hand, AT&T customer service reps that are confused about what tethering is.
Later many customers are being convinced to pay for a tethering plan when they’re in fact not tethering at all.