The SBS backup is a great tool but it really was designed to help small businesses get *some* sort of backup in an environment where they may otherwise have none. It is not particularly full featured; they leave that to 3rd parties.
In short, a NAS setup usually implies some level of systems administration so the assumption is that a more complete backup strategy will be implemented separate from SBS.
Look at products like BackupAssist or the higher-end backup products to fill this void.
