I bet the reason why billing is set up that way, is so that people forget to cancel it. Most people have high credit car balances and some people really do not analize their bill, like I do. I bet they are hoping people forget to turn it off, and they do not see the additional charge, as their balance is already high and some people may just mistake it for a fee. I know some people who only look at the balance and the minimum payment each month - they do not even look at the individual charges on their statement. i had to show a couple of people how they were getting ripped off with interest, late payments, and other fees they did not know about. They did not know that their $100 payment, only $30 was going toward the balance. They just assumed they bought something that month.
Also on that screen shot. Is the Cancel, just cancelling you out of the screen - or is that a way to cancel your 3g service? I bet many people will get fed up with people overwhelming the call centers and thus being put on hold, they AT&T is betting on missing the auto-renew date and thus squeezing another month out of people.
If your not careful - this could get tricky for some people.......