You mean this kind of friend?

Microsoft DOES do it, just not with Windows, but with the Xbox 360. When a JTAG exploit was found by free60, Microsoft fixed it just before they released the exploit to run homebrew and unsigned code. The fix? Use a new bootloader and blow a fuse on the CPU dye that was part of the old bootloader, nullifying the exploit. Unfair and shady? Given the possibility of a real brick if the dashboard update failed, you betcha. Illegal? Not so far as I've seen.