Information
Single user mode is used for recovery when the system detects an issue during boot or by manual selection from the bootloader.
Rationale:
Requiring authentication in single user mode prevents an unauthorized user from rebooting the system into single user to gain root privileges without credentials.
Solution
Edit /etc/inittab and set ~~:S:respawn to ' /sbin/sulogin ':
~~:S:respawn:/sbin/sulogin