"Fair" is somewhat loaded. The developers certainly have a right to change their product and charge for it, but it's not nearly as cut and dry in my opinion. How many contributions were made because of the completely open nature of the product? Is it "fair" to those people that the controllers of the project want to change how it's offered at a later date? Some people are happy to feel like something they have contributed is in use by a lot of people regardless of whether someone else is making money from it.
There are often lots of entangled assumptions in open projects like these. Ultimately, people have a right to offer their work as they want, so I see no problem with projects trying to request additional restrictions on how their work is used, but I also don't see a problem with companies using open projects as offerings. It was offered for free, and it's not like the cost of the offering isn't usually just the cost of the underlying resources plus some additional amount for ease of management.
AWS revenue is about $90.76B, though most of it isn't from Redis, I'd assume. But let's be generous, and assume 10% of that is. So about $10mm. For the recent version, Redis-the-company contributions to Redis-the-software were less than third of the code base, so let's say they get $4mm. That's very little revenue for a company that has a valuation of over $2B.
Considering Redis raised about $350M, $11M valuation is minuscule compared to the expected valuation to make that investment worthwhile.
The vast majority of these projects didn't seem to have a problem with large companies like Netflix using their software, even if it was put on a cloud server, as long as it was managed by Netflix. Now that the management portion is moved to the cloud provider, along with some amount of possible profit, it's a large problem? Was it not a large problem when the companies were using these projects directly? Was there not some assumption and hope these companies would use these projects by the people contributing to them?