GPL is copyleft, they're mainly doing additions and modifications to code that is GPL. They have to publish their code. I believe they try to be sneaky and hide how they add it but it's all open source.
The support contract between RedHat and the subscriber isn't anything to do with the GPL. That's a separate document. They could terminate your support if you buy a red car. They can't stop you from buying a red car (distributing the source to RHEL) but they can terminate your support and perform no further distribution of GPL'd software to you.
Terminating support contracts for using your rights could easily run afoul of other laws in different regions as anti-competition. It only takes 1 region to take this approach and the code is out.
Making a support contract dependent on restrictions on distribution sounds a lot like imposing additional conditions on the GPL, just with extra steps.
I don't buy that. They're not restricting their subscribers from distributing the GPL'd code. They're restricting who they choose to offer support to. They're not adding any conditions to the GPL. You can exercise your rights under the GPL. You just may not ever get support from them again.
Actually I think (I don't know that it's been tested in court) that this is a known security vulnerability in the GPL, as long as they only decline to do any further business with that customer (including renewing their subscription), rather than reneg on a already-agreed-to deal.
