If you want a concrete example, take Redis. (and it was longer ago, but I think I recall something similar with mongodb)
Redis Labs offers a hosted cloud offering, Redis Cloud Pro. They literally run off of AWS, as near as I can tell just selling managed EC2 instances, with replication etc. Then Amazon launches ElasticCache, which is running apparently unmodified Redis code, with AWS-developed management software on top.
Redis changed their licensing following that launch, for mainly the same reasoning as Sentry mentioned - Open-source company sells managed hosting, uses that money to further development of the open-source software. AWS (or Azure, or Google) launches a competing service, contributes nothing back to the open-source software. It's not healthy for the long-term prospects of the community (but it's great for AWS investors bottom lines).
Redis also continues to be BSD licensed.
For even more nuance, Redis Labs wasn't the author of Redis, they only sponsored the author. They weren't the first sponsors either, and nothing's stopping others from sponsoring Redis either, as long as it continues to be BSD licensed. That's the point of Open Source, that anyone can come with even just a brick to add to the cathedral, and everyone enjoys the benefits from the addition of that brick.
With closed source software, that freedom is gone.; all progress now has to go through (and be owned by) just one entity. I wouldn't want to contribute to a BSLed project because I'd be a second-class citizen in it. And even I did put out a BSLed patch to a BSLed project, the project owners can't use it for as long as I retain the copyrights on my patch.