Here's the company's explanation of the change:
In line with common industry standards, our network management practices assure that our network resources are used for the benefit of all our mobile broadband customers especially during periods when network demand exceeds available network resources (also known as "congestion").
As you would expect, these network management practices have continued to evolve over time to benefit our customers and take advantage of the billions we have spent to expand and augment our networks. As a result of this evolution, we recently revised our practices such that Unlimited Data Plan smartphone customers can now use 22GB of high-speed data during a billing period before becoming subject to network management practices that might result in reduced data speeds and increased latency.
As always, Unlimited Data Plan smartphone customers will still have the comfort of knowing that, no matter how much data they use in a billing cycle, they will continue to pay a single monthly flat rate. That is the essential promise of the Unlimited Data Plan, and we are pleased to continue honoring that promise. Further, speed reductions will occur only when the customer is using his or her device at times and in areas where there is network congestion and only for the remainder of the current billing cycle after the customer has exceeded the 22GB data usage threshold.
We will notify customers during each billing cycle when their data usage reaches 16.5GB (75% of 22GB) so they can adjust their usage to avoid network management practices that may result in slower data speeds.
The change comes following an FTC announcement that it plans to fine AT&T $100 million for misleading its customers about unlimited mobile data plans. The FCC’s investigation alleges that AT&T severely slowed down the data speeds for customers with unlimited data plans and that the company failed to adequately notify its customers that they could receive speeds slower than the normal network speeds AT&T advertised.
Read More [via 9to5Mac]