It's more underhanded than nasty but you're absolutely right that this didn't mean what they portray and most people thought.
Regardless of whether a given user moderated his data usage voluntarily, because he was throttled, or switched plans, the vast majority of the top 5% likely leaves the pool every month. This means that six months after the change was introduced roughly one quarter subscribers will be removed.
Why would AT&T do this? To avoid the negative publicity associated with eliminating a plan so many people valued while maximizing profitability. This was quite cleverly executed: Offer cheaper limited plans to save customers money, which has the effect of offering new customers a more competitive price while existing customers will happily pay the higher price as insurance against potential overage fees. Then, by removing the highest data users, they could maximize revenue from them while continuing to collect the higher fees from the lower data users. Then once unlimited becomes a less attractive offer than the $25 plan, eliminate it to avoid revenue leakage. The strategy might not be optimal but its superior to the simpler alternatives in the short run.
Of course, the reason is works is that the majority of customers are sufficiently credulous to purchase an implicit insurance contract that AT&T never had to honor. Their actions are legally defensible since the customers did receive the uncertainty reduction in the present period but most of them likely knew they wouldn't go over and paid only for the option of maintaining that protection in a future period. That said, communication that is intentionally ambiguous so as to benefit from misunderstanding should constitute an unfair trade practice.
Disclaimer: I have no knowledge of the actual thought process behind these decisions.