Doesn't the Fire (and supposedly the new Fires as well), only have the Amazon App Store and don't officially work with Play? And from what I've used on the phone version of the Amazon App Store, your app is tied to that app. If you uninstall the App Store, your app will break. So yes, I believe that is a case for lock-in.

That's more likely because Google won't let them add it, not because Amazon wants to block it. Google Play is a proprietary service and they like to use it a a threat to force OEMs to cooperate. I don't see Google allowing it on a heavily customized OS that's barely "Android" at all.

It's up to Google to allow Play. It's part of the non-open portion of Android along with stuff like Google Maps.

I highly doubt you'll see the Kobo bookstore on the Amazon marketplace; that's lock-in

Why didn't you take the 30 seconds and check? It's very much on the Amazon marketplace and works fine on the Kindle Fire.

http://www.amazon.com/Kobo-Inc-eBooks/dp/B004SIEZ8I/

iOS is much more of a walled garden, yet you have Kobo, Amazon & others living alongside iTunes/iBooks.