Well, good luck if you own an Asus with the key embedded in the motherboard.
After some weeks of being a squeaky wheel, MS sent me 8.1 one-off deal for $45. Asus or Worst Buy wouldn't or couldn't help. Actually, Worst Buy would if I paid nearly as much as a new OS.
Many US computer shops sell OEM Windows with an assurance that it's all okay. Big name net sales do the same. I have to agree that reading their small print seems to prohibit it, though I'm not sure how the home build sales would survive if the OEM copies suddenly became unobtainable.