It would indeed be a crazy setting to have, that's why no BIOS has it. You have obviously misunderstood what it does.
"PS/2 Keyboard Power On" in the BIOS has nothing to do with the keyboard's ability to access the BIOS settings.
That feature simply enables or disables the keyboard's ability to turn on the computer without you having to press the power button on the system case.
Whether that feature is enabled or disabled, you should still be able to access the BIOS settings.