Earlier quoted context omitted.
afaik the user doesn't pay for MongoDB, you can use it freely, read the code, modify; if I understand correctly SSPL is basically AGPL/GPL but with clause that cloud vendors that want to use this product to provide a SaaS version must pay to the maker. I don't think this is any unfair esepcially if this project was made from scratch by the maker and not a fork off another OSS project.
Cloud vendors are "users", too. I'm not saying it's fair or unfair, just that it's not free. MongoDB is of course within their rights to charge to use their software, but they're not somehow being exploited by those who profit from it without paying — after they offered it for free! — when they themselves profit off the labor of the same people without compensating them. Put another way: Amazon is a corporate member…
For the case of Amazon sponsoring linux, it's in the very interest of Amazon that Linux stays alive and active because all their billions from AWS comes by running linux kernel as infrastructure, not to mention having the advantage to influence the components being added and maintained that result directly in more profit for them. Linux foundation is totally different in its structure and goals from a for-profit company like Mongo.