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This argument ("it's not implemented because it's to difficult to do properly") was debated ad nauseam in the past. Well, look how others implemented it and you will see how to do it.

A simple example: S3. Currently, what happens when you exceed your credit card limit is that they send you an email they weren't able to charge you but they continue to provide the service for the next two months, and during that time everything works fine: you can access your buckets and you are charged for storage and transfer.

Now, what would happen with hard caps implemented? You didn't pay, so you're locked out of your account. Nobody can access your S3 objects, including yourself. If you care enough about them, you need to make a payment and settle your account. If you don't do it within one month, the whole content will be deleted.