Hi,
Why would you expect a username to travel in an encrypted fashion over the wire? If you were to look over RFC 3414 you shouldn't see any reference to usernames being encrypted on outgoing packets. Have you been told otherwise?
Moreover, assume a device has several SNMPv3 credentials configured (each with its own set of permissions). How would the USM on the device know which credentials you're authenticating if it can't derive the principal who originated the request?