The cloud providers have enough resources to provide APIs that are compatible with the popular ones. They won't be selling you Mongo but a Mongo-compatible replacement built in house.
Amazon isn't in the business of creating and promoting new software to developers. Not in the way Mongo/Redis/Elastic are.
"Popular things" have to come from somewhere, and my optimistic side kinda hopes that _maybe_ these new "OSS unless you're a cloud provider" licenses will be a way to fund development of software too big and complex and ambitious that it won't ever get started as a "scratch your itch" project. I dunno if that'll pan out. But I'm glad someone is trying.