Morally, it seems very high handed of MS to say that you can't change your mother board.
Just think about your statement logically for a moment.
The motherboard
is the PC.
Without the motherboard, the PC is just a pile of metal. A hard drive on it's own means nothing as it requires the motherboard with it's BIOS etc to boot off the hard drive. The motherboard is biggest single driver of PC spec due to it housiing many of the major performance features (e.g. CPU etc.)
Therefore Microsoft are quite right when they say you can upgrade your hard drive, graphics cards, memory to your hearts content, but not your motherboard.
Replacing a broken motherboard is, of course, generally another matter, particularly if done under warranty.