Namely, that if you have a company that offers something like a database as a service, search as a service or some other API, and plan to support it's development by offering your own managed service - DO NOT OPEN SOURCE IT WITH PERMISSIVE LICENCES, AMAZON WILL UNDERCUT YOU. AND THEY WILL ALWAYS DO IT CHEAPER BECAUSE THEY DONT HAVE TO FUND FURTHER DEVELOPMENT.
Take an example what Amazon did with Mongo - First they launch a closed-source competitor (totally fine) but then they also take Mongo's source and rebadge it as their own fully-managed DocumentDB (Mongo 1.6 but with closed source additions). It's having your cake and eating it too. There's nothing legally wrong with this, but I do think this is market abuse (because they can only do this because of AWS market penetration) just to make the worlds richest man richer.
From what I can find online, it looks like DocumentDB is actually a layer on top of the AWS Aurora PostgreSQL offering. This means they created a MongoDB compatible API, but are not offering a hosted mongodb cluster. (https://news.ycombinator.com/item?id=18870397)
I'd not consider that actively developing a product and adding new features.
Elasticsearch has 35000 pull requests merged. Amazon did 9 of those giving them a contributor ratio of 0.026%.
Amazon's contributions to Elasticsearch and Kibana are purely symbolic. This makes their claim as new stewards of the projects even more laughable.
Lots of companies offer S3-compatible APIs, for instance. They compete not on the API, but around reliability, price, features.
Of course, we have Oracle v Google coming up in the US Supreme Court, so things could change, but this is the way things are now.
Everybody should be allowed to change its mind when presented with new data or circumstances.
Clearly, as you say, there's nothing wrong here legally. By choosing this license, Elastic explicitly allowed anyone in the world to do what Amazon is doing now.
Ethically, I can't find anything wrong about this either. As far as I'm concerned, there isn't a "spirit of open source". Open source is a way of doing business. Elastic made a business decision. The company has to live with the consequences.
One could certainly argue far too much power is getting concentrated with Amazon and other web giants, but that's an entirely different subject...