From TFA:

Some commentators are pointing out that it's possible to sign up for a free Red Hat Developer account, and obtain the source code legitimately that way. This is perfectly true, but the problem is that the license agreement that you have to sign to get that account prevents you from redistributing the software.

So although the downstream distros could still get hold of the software source code, they can't actually use it. In principle, if they make substantial modifications, they can share those, but the whole raison d'être of RHEL-compatible distros is to avoid major changes and so retain "bug-for-bug compatibility."

Of course, they could take a "publish and be damned" attitude and do it anyway. At best, the likely result is immediate cancellation of their subscription and account. That could work but will result in a cat-and-mouse game: downstream distributors continually opening new free developer accounts, and the Hat potentially retaliating by blueprinting downloads and stomping on violators' accounts. It would not be a sustainable model.

At worst, though, they could face potentially getting sued into oblivion.

ETA the full context.

Idk, I don't think they're trying to kill downstreams. IMHO, they're just cleaning things up. Why should the RHEL source be in the CentOS repos?