A lot of billing systems are organized around event delivery. The system does what it does and reports usage. This reporting is asynchronous and can be done E.G. via cron jobs running on a 24 hour cadence in certain cases. There's an internal guarantee that billing records for a given period are delivered by a certain time. Nobody checks whether the user has enough money to do what they're trying to do, just whether they're authorized to access the system in the first place. Shutting down accounts due to non-payment is more of an abuse / fraud concern, and happens long after the bill is delivered.