well importantly, Elastic was originally Apache-2.0. Then Amazon started using their stuff and they relicensed to their not-really-FOSS-license. Then Amazon forked their stuff, a bunch of legal conflicts happened. Now they are friends again. And now they relicensed to AGPL (which is FOSS).
So they were originally Apache-2.0 which was permissive versus now they are AGPL which is copyleft.
The important distinction here is that if Amazon was to use Elastic directly, they'd have to make their contributions available to users and those users could then upstream those contributions back to Elastic. In the old situation with Apache-2.0, Amazon could take contributions from Elastic but then they kept them themself for the most part without up-streaming.
This forces a give-and-take relationship vs a one-way relationship.
Also importantly now that Elastic is AGPL they can integrate anything they want from OpenSearch's Apache-2.0 licensed projects but unless OpenSearch becomes AGPL as well, they can't pull any contributions from Elastic.
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).
So they were originally Apache-2.0 which was permissive versus now they are AGPL which is copyleft.
The important distinction here is that if Amazon was to use Elastic directly, they'd have to make their contributions available to users and those users could then upstream those contributions back to Elastic. In the old situation with Apache-2.0, Amazon could take contributions from Elastic but then they kept them themself for the most part without up-streaming.
This forces a give-and-take relationship vs a one-way relationship.
Also importantly now that Elastic is AGPL they can integrate anything they want from OpenSearch's Apache-2.0 licensed projects but unless OpenSearch becomes AGPL as well, they can't pull any contributions from Elastic.