According to the FTC post on the matter:
Ultimately, the FTC is alleging that AT&T has misled customers, because while "unlimited" should mean unlimited, it actually doesn't under AT&T's rules. Instead, AT&T has put restrictions on what are supposed to be unlimited plans, with the not-so-subtle plan to get people to switch off of those legacy plans and onto newer data tiers. The reason the FTC believes it has a case is because AT&T instituted these throttling policies after the fact, effectively changing the terms in the middle of a contract.
Not surprisingly, AT&T doesn't agree with the FTC's view of things, and is using some pretty creative excuses as to why what it has done is legal. First of all, as is the way of an entrenched power, AT&T went with the classic "everyone does it" defense (which is the same excuse you'd get from a cable company if you ask why you can't simply pay for the channels you want, or buy your own cable box), and followed that up with the very interesting defense that the few customers who were affected were notified by text, and also should have seen one of the "nearly 2,000 news stories" about the changes.
It is hard to argue that customers didn't know about the change, but according to the FTC, that's not the point, because you can't change a contract with a national press release and SMS message.