my understanding from reading this kind of threads is that there is no real way to enforce it and the provider makes no guarantees, as your usage can outpace the system that is handling the accounting and shutoff
to be fair, im not sure its a conscious choice, since its not really easy to couple lets say data transfer bytes directly to billing data in real time, and im sure that would also use up a lot of resources.
But of course, the incentive to optimize this is not there.
