Most of their money is coming from larger customers. So the small customer afraid of going over budget isn't a major concern.

They'd also rather give refunds for accidental overages than build in billing shutoffs. There are two reasons for this:

1) Their infrastructure doesn't have realtime tracking for usage so they can't detect you're over budget until some time after you've surpassed it already.

2) There is a decision that most customers would rather go over budget when something happens than have their services get shut off.