While I'm sure it can run, it's likely that Apple doesn't want to write drivers for that machine for 8. Not entirely sure why. It's most likely they feel your machine is not powerful enough to provide a satisfactory experience running Windows 8.
Now, that said, once you've downloaded the Win 7 drivers, there's nothing stopping you from putting in the Win 8 disk and installing it instead. I've done that with my MacBook Air 2011. It also doesn't support Win 8, but it works just fine with the Windows 7 driver package. Just know that if something goes wrong, Apple won't be able to help you.
Pretty much because they just don't want to is my guess. There isn't any hardware related reason for it, because it does run it just fine; I did it a while back just to see.
See post from another thread Windows 8 MacBook
I took a picture of it on Instagram when I had it up and running.
The drivers were the fun part, most of the Win 7 ones from Apple will work, but I did use specific Win 8 drivers for some however, and had to manually install a few. Nothing too major.
I've dual-booted Windows 8 on my MacBook (same make as yours) and it worked just fine with the Win7 drivers I had installed from when I used Win7 for a game (SimCity to be exact, before it was cross-platformed to OS X).
I only dual-booted Win8 simply to see for myself what Win8 was all about. I no longer have a dual-boot system set up on my MacBook, although I am thinking about dual-booting Ubuntu 12.04.