This is an interesting window into their business model. This could be a purely an altruistic decision, which businesses sometimes do, contrary to popular belief. More likely it's a bet that wider adoption from making the clustered version free will drive more revenue through their managed database as a service offering. Which shows that their non-OSI open-source license is actually leading to more code and features…
If I build my product on such software, host it on the official service, and latter their business model changes to no longer be a good fit for my business model (or if my model changes), I am stuck hosting the fork myself- I can't pay some one else to host it anymore. Now lets say that I am not alone- but there are several other customers in this situation- we can't band together to create a fork with a thriving, healthy community, including hosting options.
Avoiding vendor lock-in is my number one reason for choosing Open Source software! If your non-open source "Diet" Open license license leads to vendor lock-in, it is not much better to me than any other proprietary license.