I had never heard the term postpaid. If I'm prepaying my bill why isn't that more worthy of extras, after all they've got use of my cash? Why isn't this better than simply paying after I get the bill also known as postpaid?
Postpaid contracts tend to offer a large variety of services and options. The bill is a direct result of the usage during that billing period, including fees/costs for excessive usage.
Prepaid contracts tend to offer a minimal amount of services that are paid up-front. If customer usage exceeds what they paid for, the service is cut off. The carrier has no duty to provide service beyond what the contract states, and indeed the point is to remove that liability from the customer as well.
Postpaid is not better for the consumer, and prepaid is not better than the company.
Postpaid billing generally has a much larger range of services and features that are billed depending on usage, not contract.