The NSIS installer does no such thing, nor the the MSI require Feel free to look at the code. According to that screen shot it is Windows Installer that initiates a reboot, so it's a bug in Windows Installer on your specific system.
I can verify that using the Windows setup application on Windows 8 also resulted in an announced and unexpected system restart. I'm sure this is not a "a bug in Windows Installer on (my) specific system", but rather by design. While I don't feel anything near the disproportionate outrage as the op, it is still something that shouldn't happen. I would think that at the least there should be a warning beforehand, although it would be better if the triggering conditions were determined and managed better.
And, yes, FileBot is an outstanding application for which I am grateful! Thank you!
It's not happening on my system, and I'm specifically setting /norestart when calling msiexec. The NSIS installer is definitely not initiating a restart, it's the MSI installer which I can't control.
Can you reproduce that with every install? If it's reproducible I can try some things, otherwise can't do much about it.