That's an unfair statement in the great scheme of open source. When you fork the Android Open Source Project you only get a backbone version of the OS. You start at level 0. Google keeps all its custom proprietary and key technologies closed (rightfully). Amazon is keeping its customization closed as well.

As far as contributing back to the AOSP, that's another area where Google's iron grip on Android shows. You can report bugs and help to patch things, but there's no actual way to contribute anything meaningful to the roadmap.