Earlier quoted context omitted.
Note that these are not instantaneous. You can still incur charges before the budget action kicks in and terminates your resources.
Ok so how would this work in any other way? Should _every_ S3 action have a `if will_incur_charges() and should_not_incur_charges(): raise Exception()` statement in it's critical path? No, of course not. Everyone get's slower for nobodies benefit. It has to be delayed. But then you run into an issue: what if you end up costing AWS 100$ before the budget action kicks in. Should you not pay that? Why not?
We're talking about one account-wide flag `has_exceeded_billing_limits`. Changes are infrequent, and can be pushed into caches. Small overruns while the flag pushes are trivially eaten by AWS.