Google doesn't have a subscription streaming service like Apple, so there is less motivation for them to get their GoogleTV app everywhere. You can rent/purchase movies from GoogleTV app but if someone has an AppleTV device surely they're far more likely to use Apple for this, so Google probably thinks there's no value in having it on AppleTV. As for playing previously purchased/rented content - I think you can do it via YouTube app. Airplay is another option of course.