1. MinIO is a business and they don't owe anything to anyone for free. 2. People using the OSS version also are free to express their dissatisfaction.
This is not contract law though. This is about using OSS as a marketing gimmick to get mindshare, penetrate the market and then do a bait and switch.
From one hand, it is within their right to do whatever they want as marketing. From the other hand, we as the community should be more aware of OSS as marketing vs OSS as we would like to see it.
There is a damage to the community however: this erodes trust in OSS companies, so just like "content marketing" or "influencers" or any other type of marketing, after a while it loses its effectiveness, to the detriment of real "content", real "influence" and real "OSS".