There is no practical way to prevent people from doing this. In fact it would create a nightmare for businesses that buy one copy and purchase thousands of licenses. MS' update web site cannot track this either for similar reasons. No they cannot use MAC addresses or IP Addresses... these things change.
However, 1 purchased copy/license on 3 systems does violate the license agreement. Not passing judgement, but this is a simple fact. A fact that extends to the majority of software published, including games. This is not new and in fact has existed for years in those lenghty license agreements that 99% of people don't read.