Most AT&T customers do not go anywhere near 100MB of data and are perfectly willing to pay a flat $40 monthly fee. By cutting their bill by $30 you have just thrown away $30 of AT&T's profits. You're only hope would be to recover that money by raising the prices on the high bandwidth users by the same amount or more. If anything, by restricting their bandwidth usage you'd actually be encouraging a saving behavior that by definition results in lower profits for you. You're also cutting the profits on your largest subscription base, all for a dubious increase in "goodwill". Maybe it would be a lot more cost effective to just build more towers.
Whenever I hear tiered pricing, I never imagine a $30 discount to the low level users. I see a $5 discount (in return for a 50% lower effective usage cap) to those guys and a $20 increase to everyone else.