There's the security aspect. Windows makes you enter your product key to verify it's a legit copy. There's no such mechanism in Debian. How do you know your Debian install isn't pirated?
Pirated as in with viruses? You can enter serial in pirated windows with viruses.
You can download both from anywhere, but if you have bought serial licence key then you would download from windows website right? Yes, but you will still get windows viruses with time.
But since debian dont need to pirate since its free, then you will just download it from debian website. You cant pirate that whats its already a free copy. And you cant get windows pirate viruses in linux like debian