I would think you don't have a leg to stand on. I'm amazed you got a $60 credit. Why did you assume the e-mails were a "mistake" instead of following up on them? I guarantee the usage limits are in your (or your mother's) contract. Every service I've ever heard of does this, even cable companies.
On edit: Btw, I have AT&T, too, never had a problem except for the first month with my new phone when I thought I could get by with their cheapest data plan. Even then, they started sending regular warnings when I hit 80% of my limit, with an option to upgrade on the spot via the text message. As the others have stated, no charge for wifi use, and there is a button on my phone giving pretty detailed usage stats. Not much excuse to be "surprised" by the charges.
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