Because they don't have such a list. And what would happen if they a made an error and deactivated official OEM computer, like it happened once with XP and it's buggy wga?
M$ can't afford to lock out thousands of legit users. Could you imagine the backlash M$ would have to deal with? Lol
You say M-$ don't have such a list. But it's easy to compile such a list for a new release like Win7. e.g. WGA detects SLIC, but mobo is Bubbabyte BB-789, which is not on their whitelist. User gets BSOD or WGA nag. Easy?
I dont think they can read the bios files with WGA (otherwise they used it in Vista already). Even if they would you can always cr**k WGA like xp. So at the end we are still on the winning side
What's stopping them is the risk of screwing over millions of legit customers for the sake of stopping a few thousand "mad scientists" like us. The PR backlash from that would give Apple another thing to squabble about in those idiotic commercials. In this economy and what with the image that the public has of Vista, MS don't need to dump more fuel in the fire. The most that will happen is that each service pack will disable old versions of whatever lo*der that has become popular. But of course, by the time the new service pack is officially made available, a newer version of that same lo*der will already be out.