There's a difference though. Prior to the relicensing, Amazon had a private fork they were running and weren't contributing upstream. Only after the relicense did they open source their fork.
Now you have two open source projects and one has a copyleft license and the other has a permissive license.
Taking contributions from the permissive to the copyleft project isn't ripping off contributions. It's using open source software and collaborating in the FOSS ecosystem. And Amazon would be free to pull contributions back the other way just as well as long as they agree to the mutual terms of the AGPL (which is by all means a FOSS license).
Now you have two open source projects and one has a copyleft license and the other has a permissive license.
Taking contributions from the permissive to the copyleft project isn't ripping off contributions. It's using open source software and collaborating in the FOSS ecosystem. And Amazon would be free to pull contributions back the other way just as well as long as they agree to the mutual terms of the AGPL (which is by all means a FOSS license).