MS is to be commended for creating a good, free AV program. The impetus is most likely that Windows is highly susceptible to infection, and MS takes the heat for that. So they said "Let's provide a good, free alternative that will protect our customers, and take some of the heat of us as well". They also go after the bot and spam networks as well, and have had major impacts, although they are soon replaced by more of the same.
They took that further with Windows 8, built it right in to the OS, and turned on by default, getting updates through windows update. Gets all those slugs who can't be bothered/don't know any better/don't care covered with sufficient protection. I'm not going to say it is the ulterior motive that drives them, it really doesn't matter as this free AV program is there, whatever the reason. If they keep it up to snuff, and it protects as it should, and plays well with the OS, as it should, it's all good.