The
BIOS know perfectly if the computer needs a keyboard or not - If you've told it it needs a keyboard, it pops up the error; If you've told it it doesn't, then it doesn't. It only does what you tell it to, and if it's
misinformed, it's not its fault. The assumption is that the user knows more about the role of the computer than the
BIOS or operating system put together.
The
operating system, at least as far as the
BIOS is concerned, can't even be relied upon to exist, let alone deal with missing bits of hardware. If the machine is also missing a
reset switch, and the keyboard error is something more serious than it not being plugged in, there may not be a way to reboot the machine cleanly. Much better to halt it before the OS gets a chance to mess things up.
Also, it's a great way to test if the keyboard error has gone away yet...