I don't feel like RedHat had to do anything to sell support contracts in this case, because that was already their business. All they had to do was say they'll include container support as part of their contracts.

Correct. Maybe starting with RHEL7, Red Hat took the stance that “containers are Linux”. Supporting Docker in RHEL7 was built-in as soon as we added it to ‘rhel-7-server-extras-rpms’ repo. The containers were supported as “customer workloads” while we docker daemon and cli were supported as part of the OS.

   What they did do, AIUI based on feedback in the oss docker repos, is those contracts stipulated that you must run RHEL in the container and the host, and use systemd in the container in order to be "in support". So that's kind of a self-feeding thing.

Not quite right. RHEL containers (and now UBI containers) are only supported when they run on RHEL OS hosts or RHEL CoreOS hosts as part of an OpenShift cluster. systemd did not work (well?) in containers for a while and has not been ever a requirement. There’s several reasons for this RHEL containers on RHEL/RHCOS requirement. For one, RHEL/UBI containers inherit their subscription information from their host. This is much like how RHEL VMs can inherit their subscription if you have virtualization host-based subscriptions. If containers weren’t tied to their host, then by convention, each container would need to subscribe to Red Hat on instantiation and would consume a Red Hat subscription instance.

https://access.redhat.com/articles/2726611