Why does a pre-pidded installation of Microsoft Expression Studio 3 ask for a key?

Why does a pre-pidded installation of Microsoft Expression Studio 3 ask for a key?

I've downloaded Microsoft Expression Studio 3 from MSDN and installed it. However, after installation, I start the app and it asks for a valid product key, despite the fact that it's pre-pidded (e.g. requires no keys, or so the MSDN download page says).