So are Red Hat source packages still available? What incentive does IBM have to make them available? The source code of the applications, sure. But why the source rpms? Why doesn't Amazon and friends just make a dist that is not tied to Red Hat?
Because RHEL is a known distro many vendors test for and allow. Having compatibility with it is a huge boon for adoption, since you can just use lots of software. If you do a distro from scratch, you're on your own.