You cannot log into an insecure tty(4) as root.
This shouldn't matter, as the best practice on the BSDs is to sign-in as a normal user and use su(), sudo(), or doas() whenever superuser authority is needed.
If you need root access directly from an insecure console, then single-user will get you there.
The NetBSD ttys(5) man page says:
Code:
secure If on is specified, allows users with a uid of 0 (e.g. "root")
to login on this line.
OpenBSD's man page has a little more descriptive text, circa 2008, which your experience indicates is applicable to NetBSD:
Code:
secure If on is also specified, allows users with a UID of 0 to log in
on this line. If set for the console entry, then init(8) will
start a single-user shell without asking for the superuser
password.