It's the same model than redhat, you buy a subscription, and the software checks if you have a valid subscription. Nothing is stopping you from modifying the source to remove that check, re-build and re-distribute for free (or for a fee) under another name. This allows community-driven projects like Centos or Fedora to exist. It's an ambitious business model, which doesn't prevent other to compete. The bet is companies that rely on the service will want to pay and will prefer getting professional services / hosting from the original maintainers.