There's nothing to stop them, no. But that wouldn't change that they have to make the sources available to their customers -and it takes one customer to get the source and provide it for a fork.
I guess what I am trying to say is that perhaps it's an inherently flawed model as evidenced but what just happened and, going forward, we should rid ourselves of such ties.
At the end of the day, we're talking about a super stable server distro with LTS. While many things are easier said than done, I believe this is important enough to the community that it could happen. My main concern with such a distro is security, a team developers providing security patches would be the most critical part of such an entity.
Hopefully before CentOS 7 goes dark in 2024 there will be viable alternatives. Right now, having to compile against custom libs not shipped with the distro (for example, modern OpenSSL) is a small price to pay for having another 3 years left to decide on best course of action to get off CentOS. Still, it's a terrible position to be in for those who upgraded to CentOS 8 already (as we were just about to do).
But if getting caught making further distribution of RHEL causes a Customer to lose their support contract then the Customers would be unlikely to do so.