This is basically AT&T indirectly saying their network sucks and that they'd rather you not use the data you paid for.
Here's an analogy that I hope makes sense: Consider a buffet restaurant. They advertise "all you can eat" and base their food supply and equipment purchases (dishes, refrigerators, etc.) on an expected number of customers and an expected average meal size. If they run low on food as customers show up, their meal size prediction might have been low and/or their business might be doing better than expected. Either way, they need to adapt, e.g., buy more refrigerators. If they don't, they're hurting both their customers and their business.
If some customers are eating way more than others, they could try to stop them on the spot, and probably get a lot of bad publicity. They could raise prices to cover the true average. Or they could consider switching to tiered pricing. If they choose tier levels based on the weight of food in a meal, they are bound to get complaints, but it's not unfair. If they noticed that most men ate more than most women, they might think it was fair to charge men and women different prices, but they'd get in legal trouble. A more acceptable approach to tiers might be to raise the basic price for most people while giving discounts to kids and seniors (who are unlikely to be overeaters). That's fair too.
The equivalent to your statement would be that the restaurant says that their food supply sucks and that they'd rather you not eat the (potentially infinite) amount of food you paid for. The first statement could very well be true, and I'd expect them to work on improving it. The second statement is no doubt true but shouldn't be surprising. Covering their costs, with or without tiers, and having a fair system is better than throwing overeaters out of the restaurant without warning!
Back to AT&T. If they introduce some way to charge high-bandwidth users more than low-bandwidth users it might complicate our lives but it won't surprise me any more than Apple finally accepting tiered pricing in the iTunes Store, and I won't consider it bad business practice.