Well, to be fair: there are lots of reasons for releasing and using free software. Google's (as opposed to Android's) interests aren't necessarily served by someone else rebranding it. And a project generally is hurt by a fork. Having Amazon's even more walled-off fork of Android exist does nothing to help users of smartphones.
But your broad point is exactly right: we're all better served by working together on software instead of racing for parochial advantage by building on top of it without contributing to the whole. Open Source has lots of big advantages, but to do it "right", there are costs to be borne as well. The Linux community has on the whole found a pretty good balance between these concerns. The Android community has had some pretty bad missteps.
But your broad point is exactly right: we're all better served by working together on software instead of racing for parochial advantage by building on top of it without contributing to the whole. Open Source has lots of big advantages, but to do it "right", there are costs to be borne as well. The Linux community has on the whole found a pretty good balance between these concerns. The Android community has had some pretty bad missteps.