New
#101
If you have a retail copy, nothing. Just like nothing stops you from installing that retail Windows 7/8.1 on endless machines. After a while, Microsoft will require you to call them to activate, and if, say you put the key on the internet, and a billion people started activating the same key, it would be blocked.
I really don't understand why this seems strange to you. Retail keys have always been more or less on the honor system up to a point.