Despite their claims, the throttling does little to help keep the network functional. When a sector is congested, they should throttle everyone. Instead they throttle a small group of people regardless of whether or not there's any congestion. That won't do jack to help their overall network problem
So it's not really a fair use situation
One thing that you have to realize once you start designing and maintaining really large scale networks is that nothing works in a vacuum.
That is to say, while you are correct that there are certain instances where the individual throttling may not have been strictly necessary in one regard (say, because the local RF conditions were still favorable). You do have to realize that there are dozens to hundreds of other systems that are impacted by your activity.*
Thus it is possible to throttle on a micro level (a user connected to a cell tower which otherwise isn't overloaded) when the reason for that throttling is actually a macro level event (constrained peering link, overall re-prioritization of channels to voice due to upcoming anticipated load need, need to stay clear for higher priority traffic at some choke point further upstream, etc)
Again, to be really, painfully, clear. I think it was stupid that AT&T tried to sell cellular as unlimited in the first place. It's a fine concept in wireline (although expensive to do properly). But it is unsustainable in wireless.
The fact that they have continued to grandfather the plans boggles my mind.**
They simply don't begin to charge enough money to supply "unlimited" data, so they need to drop the offering altogether.
They probably also need a punishment that while hopefully not excessive, is painful enough to make everyone understand that "unlimited" isn't something that should be offered unless it truly is without any sort or limit.***
Karl P
*In practice, that means that what is likely happening is that ATT is running their "unlimited" network just like their regular tiered network, and after the "unlimited" network runs through their back-end data allowance as a group, or hits certain tiers as a group, throttling is engaged. Some places it is engaged may need it really desperately, and in other places maybe it wasn't actually needed for micro-network conditions, but in aggregate, across the whole, it hit a certain threshold.
**In reality it does not boggle my mind - I know that they do it because they assume that a good portion of unlimited people would leave if they just unilaterally pulled the contracts. And they are somewhat correct. Thus they decided that the potential cost of a lawsuit was lower than the lost opportunity cost of people leaving. As of yet they have been correct, but it's a horrible way (IMO) to do business.
***If you think about it though - buying an ongoing service for a fixed price where there is truly no limit placed on that service is one which you would reasonably expect to pay an enormous amount of money for relative to a service which has a limit, of any type.