Accounting poses some serious problems.
For one thing, packet service is inherently ill-suited to detailed usage accounting, because every packet is independent. Maintaining detailed accounting by the packet similar to phone company accounting may be too expensive. Another accounting problem concerns the granularity of the records. Presumably accounting detail is most useful when it traces traffic to the user. Certainly if the purpose of accounting is to charge prices as incentives, those incentives will be most effective if they affect the person actually making the usage decisions.