I don't really get how people get this twisted.
ES and other companies have a business that sells a managed version of their product. This is how they sustain developers to continue working on Elastic Search. This model has worked for companies long before cloud providers were a thing. What AWS and others basically did is create identical services, keep all the profits, and exploit gaps in Open Source licensing to this end. From the ES perspective, their FOSS contributions were done in good faith which basically boil down to, "If you can run our product on your own, you get it for free".
AWS knows that if they take too much of ES' market that they won't survive. If they don't survive it will just be a matter of time before ES is dropped by Amazon and totally unsupported.
You can frame this question in terms of ethics, you can frame it in terms of licensing naivety, you can frame it in whatever way you want but Amazon is doing what it always has done: exploiting smaller businesses in its goal to become a conglomerate.
Edit: a lot of people talking about the license forget that there's an entire spirit to open source. The permissiveness of open source was one thought to be "we can all succeed together" and what people get upset about is the fact that this obviously violates that spirit. The businesses set up to back companies like Elastic Search were setup to sustain the project while continuing to empower it's creators to take their vision further. If Amazon takes the pie, that doesn't happen. At best, the creators are now Amazon employees and have to follow their desires. Just because you can exploit a license, doesn't mean you should.