Hello,
Yes, this is "by design".
But wait! Let me explain..
Let's say that you enable demo mode... and you have it up on your website for customers to try out.
And then they disable demo mode.. and start writing over your demo files, etc... or changing your demo user's password, or adding users, or whatever.. (all those things are disabled in demo mode).
That would be a problem.
So the only solution I could think of was to not allow the ability to "turn off" demo mode once it has been turned on.
The solution:
1) Download and open the /snippet-db/random-folder-name/program_settings.php file with a text editor.
2) Change line #9 from this:
demo_mode#1
to this:
demo_mode#0
3) Save and upload the file, overwriting the old version.
That will "manually" disable the demo mode.
I'll probably eventually add some way to do this without manually having to edit the database file.. but for now it's a manual process.