It's not perfectly legal, but reasonably more legal than the alternative (a key bought from a reseller). Those stickers are meant to be affixed to a machine sold by a reseller, and is only meant to be used for that machine. If the machine is destroyed and the sticker/key is kept, then it can be used, and it does register Windows Professional/Home installation rather than an Enterprise/Education installation, but is in violation of the TOS that it's provided under.

That being said it is difficult to shed a tear for Microsoft of all companies over this practice.