I ran into the same situation with my clean install with my TechNet ISO. My laptop has an embedded Windows 8 core key but I was prompted to enter a key. I think it looks at the key but if it isn't an 8.1 key it just ignores it. Just for fun I tried one of my Windows 8.0 TechNet keys and it rejected it. I had to use a Windows 8.1 TechNet key to finish the install.