Earlier quoted context omitted.
The thing that surprises people is not that Stallman went to Microsoft but that Microsoft invited him to talk. Back in the day Bill Gates would make signs to ward against evil if Richard "communist cancer" Stallman was anywhere near Redmond. How times have changed.
I am bit more cynical about this. Google and others have simply shown Microsoft how to make loads of money by opensourcing the project, while still keeping the upper hand on it. How to "opensource-but-not-quite-opensource" (Chrome/Chromium, Android/AOSP,...) the projects. Even Amazon didn't succeed in prying away Android from Google (Fire). So in my mind it's actually surprising that it took them so long to join the…
They didn't? Amazon's phone failed because it had no reason to exist—it was a new product in a crowded market and offered nothing unique. Nothing to do with Google or Android. Amazon's tablet line still uses their Android fork, and seems to be doing fine.