If you wonder why, this is the explanation:
The "exit nickel" image only works if you entered nickel from KSM. If you booted directly into nickel it does nothing.
When you set KSM to "never" start, it honors it, and it doesn't start, instead the reader boots directly into nickel.
Since KSM is never starting, there's no way to get back into it, except by changing the start setting, which can only be done by editing the file externally, as you found out (or maybe with some fancy patching).
Personally, I have KSM set to "alternate", that way I can exit KSM by rebooting, and enter KSM by rebooting
