Reality is that in areas of poor standardisation, vendors will test against Windows and ship it if it works. You could argue for a cross-vendor compliance testing suite, but if the behaviour of Windows varied from that then people would still need to match the Windows behaviour because that's where the market is.
So for all practical purposes, the behaviour is standardised. It's just embodied in the behaviour of Windows rather than anywhere useful.
FWIW, the fact that it's standardized against Windows makes Windows kernel developers' lives suck too - any change can break some dumb device that only actually worked purely by coincidence against XP SP1 or some ancient version
I think that the other issue is the culture around(and design) *nix and BSD systems, where a reboot is generally only done when very bad things happen to the system, and nothing else can correct it. Often, there are a lot of other things going on that prevent the system from gracefully rebooting.
So hardware vendors might test a "standard" reboot, where nothing is broken, and the OS is in a stable state. Unfortunately, that probably isn't the case where most required reboots occur.
The behavior is standardized to a degree that it works for Windows, but according to your article Windows does its own poking around for the reset. It would be nice if there was one established protocol for resets rather than any poking around at all.
I guess this was the point of your article though.
So for all practical purposes, the behaviour is standardised. It's just embodied in the behaviour of Windows rather than anywhere useful.