The concern is that Red Hat terminates your account if you redistribute the source to another party. This feels like an additional restriction placed on the source code, which if it is, would indeed violate the GPL.
Now THIS is a GPL-violation or at least a serious concern and asshole move.
Serious concern and asshole move? Yes. Gpl violation? Not sure. You could argue you are not restricted to do whatever you want with the code you receive with a subscription. But if you share the code, they don’t want you as a customer anymore and won’t give you new code. I don’t know if the GPL allows that.
Terminating a support contract, in itself, is not a GPL violation. The restrictions only affects the ability to receive future updates.
Edit: Red Hat indeed claims that no GPL violation is happening, yet they inform their customers that sharing updates leads to contract termination, which clearly breaches the GPL at least in spirit: https://sfconservancy.org/blog/2023/jun/23/rhel-gpl-analysis/
I think it depends on whether it’s considered an additional restriction on the recipient’s right to redistribute the software.
Saying, “you can redistribute the software but you will face _____ penalty” seems like a gray area to me.
Context is important. It’s possible that the software is distributed without any warning like that and that the termination of the support contract is done without citing the redistribution of previous versions as a reason. OTOH if the customers could prove that there’s widespread knowledge of the retaliatory termination that could be equivalent to a (non-written) restriction that is indeed incompatible with the GPL
