Red Hat doesn't need to mention it. It's a legal requirement of the GPL... anyone using the binaries has legal right to the source code.
As for Red Hat cancelling users accounts who pull the source build binaries and share it... that'd probably land them in a lawsuit, because as long as that person who originally downloaded the code has the binaries... they are legally entitled to the source for them for as long as they have the binaries.
Furthermore, anyone who shares a binary they build from RH sources... has a legal requirement to share the source onto the next person.
These are precisely the type of issues with the MIT/BSD license that Stallman wanted to address with the GPL.
Red Hat could terminate the dev license... and put RHEL entirely behind a paywall, but they have not stated that they are doing so. And I would imagine that they would get a ton of backlash if they did considering that was how they addressed the reaction the CentOS/CentOS Stream change.
But even if they did that, they still have to provide source to anyone with the binaries. So all it would take is one person buying a license, and then releasing the code.
Even if Red Hat banned that account, there would just be another account to do the same thing again. Red Hat would have to play whack-a-mole to try to stop people from doing that constantly.