I don't see why not one would build on documentDB? We can't fight the "ego" fight for MongoDB owners. Good luck to those who fight for brands even in OSS ecospace. They have to defend their baby and not use the OSS developers to do that for them. Amazon did nothing illegal. They didn't violate any GPL or anything.
Amazon AWS is the place where big players go for their cloud needs. So if documentDB is suitable for those needs then why would anyone want to use mongo? If Microsoft comes with something better on Azure, then again why not?
As long as it's open source, sure, anyone should make imitation or whatever they want to serve their customers.
Life is too short to play favorites.