When using the samba-tool dns sub-commands an admin password for the realm is required to continue. The password can be entered interactively in the console or by using the password flag in the command. This is not the case with other sub-commands for samba-tool.
The password requirement is common to many commands, and where it is needed there is no way around it: the server requires an authenticated user.
Unless I have misunderstood, this is not a bug.
Douglas is right. The DNS management is done via the DNS protocol and thus needs authentication. Operations which are done directly on samba database files don't require authentication.