My formula is not flawed. All you are saying is you could choose to reduce the time half of the equation yourself, therefore reducing the max data you could theoretically use by your own decision. But this half of the equation is true with throttling or not.
However, if you have a certain data requirement, AT&T throttling you forces you to expend more of your time to use the service. Your time has some value, which means they are increasing the cost of their service.
The other option is you choose to make the usage time constant. But that would mean using less data as a result. So in this instance, the value of the service has been reduced.
Now unfortunately you have no recourse, because it is in the contract they can do this. But you need to think about whether it is in your best interest to sign a contract that says that a company can either increase the (time) cost of a service, or decrease its value, arbitrarily based on any factor it deems valid, and give you no recourse.
The carriers should be forced in order to implement this action, to prove to regulators that without it, they would actually suffer ill-effects. And to do so on a market by market basis, so only those users who do have the potential to harm the fidelity of the network are being affected.