Perhaps because they rely on a lot of open source components and open up some of their technologies whereas Microsoft has been slow at coming around to the OS way of thinking.
If you're worried about lock-in with App Engine, check this out: http://code.google.com/p/appscale/ As you can see, Google is one of the supporters of this project, although it's not clear exactly what they have contributed.
If you're worried about lock-in with App Engine, check this out: http://code.google.com/p/appscale/ As you can see, Google is one of the supporters of this project, although it's not clear exactly what they have contributed.