Why would MS have any interest in hamstringing the sale of used consoles (mind that MS still most profits are coming from game sales)?
It may be that it's part of the DRM, that once a machine is set up with a users profile it can't be deleted easily (the HD is 'non user replaceable' too) - It'd act as a sort of anti-theft device and ensure nobody could claim it wasn't from their machine that a digital transaction was made?
If it's true, then they might be undecided as to whether you'd need to part with a machine through a 'participating retailer' or have some kind of hardware unlocking/erasing facility by remote?