Red Hat does not have any right to place restrictions on the distribution of derivative works that they do not own the original copyright for. Threatening to terminate a service agreement is a restriction.

That’s the first time I’ve heard someone argue to that point. I agree.

The arguments I’d heard were that they didn’t have the right to stop publicly distributing the sources. They do have the right under the GPL stating that the source must only be provided along with the binaries and, if the binaries are only available to subscribers, so too are the sources.