Why not allow people to set a cap that disables everything set to that billing method once it's been reached? That seems to be the use case that people are saying is missing from cloud offerings.
I imagine most of those companies can afford to have some overages built into their caps. They should also be able to afford the developer time to create fallback behavior, and graceful degradation of services.
Still, I bet there is a lot of companies that might be more upset with the provider shutting everything down even if they had set up a cap.