Because it helps adoption of their API. They will be the premium provider for their platform, and the bigger it gets, the more business they get. Plus many businesses won't consider a technology that has only one vendor, so having an open alternative opens doors.
The original claim was that users benefitted from open alternatives. Is there any reason why users don't get those benefits if the "closed" source helps provide those alternatives?
I note that open alternatives to the Microsoft stack were seen as a good thing, yet alternatives to the Google stack are somehow second class because Google helped.
Is it fair to mention that some of the alternatives to the Microsoft stack were developed with some help from Microsoft?
Why?