AT&T Cracking Down on Unofficial iPhone Tethering Users ➝

OS X Daily:

Customers are being notified that their service plans need updating to subscribe to a tethering plan, and that they will be automatically subscribed to a DataPro 4GB package that costs $45 per month if they continue to tether. In the email, AT&T also notes that if customers discontinue the use of tethering, no changes will be made to their plan.

I can understand this change for people who have the unlimited data plan, but if you are on the 2GB a month plan you should be able to use those 2GB however you choose to.