Development of postgres is funded by the companies using it, for instance Amazon.

Elasticsearch and Redis are private companies that fund most of the development themselves.

When Amazon sell Elasticsearch and Redis, they are in direct competition with its creators.

Obviously such a situation isn't sustainable in the long run, and as such both Elasticsearch and Redis (and to my knowledge also mongodb) have changed their licenses to avoid that cloud providers sell their OSS product without paying a license or otherwise contributing back.

In the case of Elasticsearch and Amazon, Amazon even used the Elasticsearch brand to sell their own version.

As I see it it's a good thing that cloud providers are forced to take part in maintaining the OSS software (forked or not) that they are cashing in on.