This isnt unjust at all. You had a chance play with, and if you chose to, test the next version of Office before anyone else. When you signed up for the beta Microsoft never advertised 'Free copy of Office 2007 to all testers!'.
I think the privlidge of being able to test the product and influence it's development is more than enough compensation, however thats not to say I wasnt thrilled to see a free copy of Vista show up in my inbox.