Cause it is an upgrade. Meaning that you had to have used a previous qualifying OS before the OS was installed. The clean install trick was never sanctioned by Microsoft. A clean install using upgrade media is never the intention of the media. Hence the term upgrade.
So while you can, doesnt mean anything. The media is supposed to be used with a previous version of Windows already installed, if the person doesnt know the trick or have a previous version of Windows, then they are SOL.
Plus, again, upgrades do not qualify for transfer. Only full retail versions qualify for transfer with the official disk and Certificate of Authenticity. If you do not have both of those items as well as deactivate Windows from the previous machine, then even the full retail version doesnt qualify for transfer to another owner.
These are the terms laid out in the EULA that you agree to at the very beginning of the install of Windows. Most people do not read them fully to know that upgrades are only to be used by the person who purchased them.