But RHEL is not free. So... can’t this be included then only used after the user sets up a license?
It's still GPL and you still get the full code, hence CentOS. The benefit to licensed RHEL is the support.
Support is not free. If you want software updates, security patches, bug fixes, or somebody to call when shit hits the fan you pay Red Hat.
If not CentOS/Fedora use the same source codes