Not only that, but I've just realised... $2.5k/m only gets you 10 times as much as a free account. Sounds like such a great deal!

And then if you want THREE times as much as the start-up account, you have to pay FOUR times as much - $10k for 30 million calls.

Maybe they should have hired a mathematician instead of geographers....

Not all services scale perfectly linearly. They are likely limited by certain costs associated with using an Amazon infrastructure or their own infrastructure, or it is simply a matter of customer hot spotting trends and perceived demand.