This move really surprises me. The people on the Windows Weekly podcast said that it is a way for Microsoft to get the next generation of creators using Minecraft today into Microsoft's fold and getting them doing programming Windows applications in Visual Studio in the future. I thought that idea was a bit out there, but who knows.
It's clear that Minecraft has been a massive success and I'm sure Microsoft is looking to somehow exploit that. Maybe it's a way to get kids thinking about Microsoft being "cool" so they grow up interested in their products in contrast to those of us that grew up during Microsoft's "embrace, extend, extinguish" phase and think of them as borderline evil. But who knows if they actually have a solid plan. For all I know, it could be: "A lot of people use this. A lot of other products license its characters. It has very positive mindshare amongst its users and parents. Let's buy it! Later, we'll see if we can figure out how to make a lot of money off it."
Edit: Grammar, clarity, additional content.