http://en.wikipedia.org/wiki/Runlevel
http://en.wikipedia.org/wiki/Single_user_mode
What a problem? SliTaz not use runlevels, its init not allows any so-called runlevels.
Here is another problem. If you installed (any) OS onto unencrypted partition, anyone can boot other OS, login as root (in that OS), mount your partition, and steal/remove/modify all your secrets, including logins/passwords. Let's say, fake root can modify hash of your root's user password in /etc/shadow to known value, and login as root into your system.
PS. First line of defense is your BIOS Setup. Disable booting from CD/DVD/USB external drives/USB Flash/Cardreader/etc. Set password to your BIOS Setup.
But BIOS Setup can be resetted, or HD can be transplanted to other machine...