Press the power button on the front on the iMac computer instead.
You can use the Apple USB Keyboard with all iMac computers, but the circuitry that enables the keyboard’s Power button is not present on the models listed above. Therefore, pressing the Power button on the keyboard will have no effect.
Then, there’s this gem:
The parenthetical product descriptions (Summer 2000) and (Summer 2001) refer to the summer of the Northern Hemisphere.
Good to know, Apple.