I can understand your frustration. But look at it from Microsoft's, or any other organisations, point of view. For charity, you can install several "Free" editions of Linux,. Unfortunately, Microsoft's intention is to make money for the shareholders.
They have no way of detecting, if, as you say, you were given th freedom to install on your "own" computers, if they were indeed yours, or passed on to other users - hence the safeguard. Should they allow that freedon of choice, there would only be A need for a few downloads to be available on the web, and only idiots would then still be purchasing the product. Windows 8, or whatever, would never happen, and Microsoft, in a very short space of time, would be scratching around selling computer utilities.
Fwiw. Your two DVD's gave you the rights to the indefinite lease of the OS, not its outright purchase. Apart from the physical costs of producing the DVDs, all you purchased was the single key, to be used on the OS of your choice.