WHY I don’t know.
But smb.service starts smbd, so that’s exactly what you want.
And nmb.service starts nmbd.
And it’s consistent with the sysvinit scripts in earlier versions, where rcsmb and /etc/init.d/smb were used.
I will need to add that to the wiki on OpenSUSE
This one?
http://en.opensuse.org/Samba
This already does say:
systemctl start smb nmb
and
systemctl enable smb nmb
(the “.service” can be omitted)