Because under the hood Windows 8 and Windows 7 are not that different. This is especially true when you compare 64 bit to 64 bit and 32 bit to 32 bit. Also OS vendor provide advanced copies of their software to the program vendors long before it is generally available to the public.
I know there area some pretty significant under the hood differences, but as My WIndows 8.1 boots directly to the desktop and from a user point of view, it runs like an updated XP machine.
The big difference is the modern interface which is a disaster on desktop or laptop computer. But as I said you can easily set it up to boot to the desktop and never see the modern interface.
While I do not have an online tree, I am running FTM 2012 on WIndows 8.1 and have had no problems. I have about 2000 people