AWS doesn't have to check that for every request. They only have to eat the cost if you go over and use more before they shut you down. And the shut down should be in a way that they switch data to read only and give you a day to react before they delete.
They might even offer this as an insurance, so you pay a little more but can be sure you stay in budget.
They might even offer this as an insurance, so you pay a little more but can be sure you stay in budget.