Actually, it goes back even further than that.
First ADB computer was the IIGS (and ALL ADB keyboards have the "power" button), and it had its own keyboard (you can certainly use it with no issues at all on any ADB Mac, but that keyboard was only sold for the IIGS). And it didn't have soft power, just a switch on the rear. It was the RESET button (the 8-bit Apple II usage model being dependent on being able to easily (partially, anyway) reset the machine with a Ctrl-Reset sequence, and fully reboot with OA-Ctrl-Reset - more on that later).
The Macintosh II, several months later, was the first machine to actually use that button for power. (Although, it was intended from the beginning as a power button - ADB even has a dedicated pin for that - just that the first Apple product to have it, didn't use it as that.) And, any ADB Mac, you can do Cmd-Ctrl-Power (or I think it even works on USB ones, even x86 - I'd try it on my MBPR, but I don't want to do a hard reboot right now) and get an instant reboot. So, that power key isn't just Mac legacy, it's Apple II legacy, straight back to 1977 (and, actually, any Apple I keyboard needed to have a reset key, too, so even though Apple didn't have an official keyboard (although they recommended a specific Datanetics part), it goes back to the Apple I, in 1976).
Speaking of Cmd, and power being the reset key... notice how every ADB keyboard, and quite a few earlier USB Mac keyboards, has an Apple logo on the Cmd key? That's not a gratuitous logo (well, it is gratuitous on the USB keyboards). It's for Apple IIGS compatibility - see, by the time the IIGS came out, a lot of software referenced the Open Apple key for keyboard shortcuts. So, when the Apple II and Mac lines were unified as far as peripherals, Open Apple became Command, Closed Apple became Option (although the logo never appeared on that key). And, the Open Apple and Closed Apple keys? They appeared on the Apple II line in the //e, in 1983, albeit implemented in a rather quirky way - they were actually tied to the joystick buttons 0 and 1 (so that nothing keyboard-related had to be changed, the Apple II keyboard subsystem really having descended from early ASCII terminal keyboards, and not the modern OS-handled modifier setup - for instance, Ctrl isn't handled by the OS or ROM, it actually shifts the entire keyboard down by 0x40 and ignores shift, IIRC - so Ctrl-G (ASCII 0x47) becomes BEL, or ASCII 0x7). But, those buttons came from the Apple /// - meaning that the Apple logo being on the Cmd key is a bit of 1980 legacy (although you could argue that it's 1983, because no Apple /// software runs on the II).