It's difficult to come up with a good model for how a billing ceiling would work in software as a service. A good start would be to fully specify what behavior you desire when an account hits its billing limit. Are you expecting everything to keep working like normal while the cloud provider pays the bill for those resources, or are you expecting the provider to fully shut everything down in a way that prevents the accrual of further costs, or something in between?
There are a number of resource types that, simply by existing, will accrue costs. A lot of them, actually. On AWS that includes things like running EC2 instances, EBS volumes, RDS databases and backups, DynamoDB tables, data in S3 buckets, and more. The question is what should happen to these resources upon hitting a billing ceiling?
Should EC2 instances be terminated (which deletes all data on them), DynamoDB tables deleted, S3 data erased, RDS databases deleted? If that was the behavior, it would be an extremely dangerous feature to enable, and could lead to catastrophically bad customer experiences. This is a nonstarter for any serious user.
Conversely, if you expect those resources to continue to exist and continue operating, then that's basically expecting the cloud provider to pay your bill. The provider will then have to recoup those costs from other customers somehow, and so this option sets poor incentives and isn't fair to others. If you expect your account to remain open the following month, you'd have to settle the bill, and we're back to square one.
AWS gives people tools to tackle this problem, such as billing alerts. These can notify you over SMS, email, or programmatically when you hit an "$X this month" billing threshold, and then you can decide what to do. Since these events can be processed programmatically, it's possible to build a system that will automatically take whatever action you'd like AWS to take, such as shutting things down or deleting resources.
If you think all of this through, it's really hard to come up with an approach to billing limits that's fair and a good experience, so I think it's reasonable for cloud providers to give billing threshold alerts while leaving the choice of what to do in the hands of the customer.