Normally, I don't like to have to think about things like licensing after a very late night in a very cold stadium watching a very hot Peyton Manning light up my team, but this really is a good one.
Even more, I would normally take the side of Mr. Regular Guy and make Microsoft jokes, but not this time. For the record, I can't stand that I have to pay for the same license, which is already too expensive, over and over again. I enjoy making "blue screen of death" quips in my speeches (it's always a cheap yet effective laugh). I love to bash the mighty giant whenever I can. It's not just Microsoft, by the way. It seems I have a Napoleonic complex and have to try to slay any big king. (I love the underdog, and with the exception of the Patriots, I hate the king.) I also am a realist, or fatalist. Microsoft is the king, and I am jealous. It is in a position to dictate how I spend money, and it is in a position to not really care whether I like it or not. I don't like it personally, but I love it intellectually.
"Because they can" is the business equivalent of "Because I said so."
So here's the justification: function vs. content. Society has created a mode where content, once licensed legitimately, is portable. Function is not. Your example of the DVD is the perfect illustration. You buy The Dukes of Hazard DVD (for the intellectual content, of course), and you now have the right to play said DVD (during Masterpiece Theatre downtime) on any DVD player, whenever you want -- except internationally because they change the format (hmmm). The DVD player itself, however, has an individually licensed set of embedded functions running on it. The DVD itself has individually licensed function within it -- Dolby Digital, for example. You own the right to view the content whenever and wherever you want, but anything you want to view it on will have the same individually licensed function.
When a movie is filmed, it is done so utilizing various individually licensed functional bits of technology. It was edited using computers and software, each containing right-to-use licenses. If you buy two Avid editing systems, you get two licenses. Microsoft is no different. It sells function. The function it sells is licensed to a machine. If you want to add another machine, which can also perform that function, buy another license. You can complain that the function it sells is too expensive, too complicated or dumber than Jessica Simpson, but you can't really complain that if you want to perform that function, you have to pay the fiddler.