The GPL clearly grants every recipient the same full rights as whoever they received something from, so copyright law itself says that you can take that source and redistribute it.

But starting a year or so ago RedHat says you may not redistribute, which they can't actually say, so you still can, but if you do you will be fired as a customer and lose all future access, so you only get to do it once.

Given the clear wording that makes the intent of the GPL unambiguous, that every end user is fully empowered and you may not do anything to curtail that, I don't see how they get away with it except the simple fact that no one has been willing to take on the legal fight.

If you're a pre-"stream" Centos user, you don't have the money for that. If you're a paying RHEL user, you don't want to be on bad terms with RedHat or maybe IBM either, and neither the licence costs nor the rules bothers you at all anyway.

So it's plainly illegal in my opinion, but will stand, illegal and yet in effect and unchallenged probably indefinitely.