As someone who admins around 200 Rocky 8/9 and Centos 7 servers, this is a little concerning.

But I have a lot of faith in Rocky and Alma, who are reportedly working together, in coming up with a solution to ensure they continue getting security fixes and updates.

Redhat are steadily turning into every bit as anti-competitive and, well, evil, as Oracle used to be. It's a shame as they used to do a lot for the FOSS world. Now they seem content to profit from it and give nothing back.

Aren’t there poison pill clauses in a lot of OSS licenses that prevent moves like this? Could they face legal repercussions?

Big Blue asserting their dominance. Unfortunately at the cost of some very fantastic community projects.

Have they given a reason? The blog post doesn't list one.

This has been coming for a long time.

Ehh, well it's something interesting. I've never really ever used RHEL myself on any of my servers. I do know some people that might be impacted.

As a strictly desktop/personal Linux user, could someone enlighten me: What advantage (if any) does RHEL have over Ubuntu Pro at this point?

They aren't going closed source though? Just not providing source to everyone. But everyone who gets binaries from them still gets access to the source code. Unless I'm missing something?

The discussion on the LWN post gives some insight into why this is probably happening. Most likely due to Rocky/Alma not contributing upstream while benefiting from Red Hat's work.