You bought XGB this month. You didn't use it all? Ooops, you overpaid. Punished.
You used more? Ooops, you pay a higher rate for the overage. Punished.
How about we treat this like any other commodity and I get exactly what I pay for?
It is pretty well established. Where your model deviates from reality is in not accounting for the mental costs associated with having to (/wanting to) track how much bandwidth you've used on a minute-by-minute basis... how much is that Netflix movie costing me? How much is this Youtube video costing me? etc etc
You can argue about how irrational this is. You can even be objectively correct. And it won't matter very much.
Even objectively, you do need to worry about some fixed costs too. A package deal for the first X GBs might now be as irrational as you initially think.