If by "what do you expect a company to do" you mean what is _ethical_, then what's good for their business seems like an entirely orthogonal concern to me.
If the question is: What is good for the continued viability of open source -- I honestly do not know.
Given that and the fact that Elastic is the primary sponsor of the ElasticSearch open source project, it is obvious that this action actually is in the best interest of open source, as it allows Elastic to continue sponsoring ElasticSearch.
Most of the criticism of Elastic in this thread seems to be predicated on the purist notion that proprietary software is the antithesis of open source software. That's obviously untrue, and we really need to disillusion ourselves of this.
The ability to fork in open source is a feature not a bug and ultimately the community of users and developers of these projects will decide what works best for them. In this case it isn't even a fork, it's new plugins that the community can consume under a FOSS license.