[simias]

I always assumed that it was a marketing decision, RedHat don't want people (especially the corporate type who buy a ton of servers but might not know the small details) to know that "RedHat, but free" is an alternative. Instead we have "wait, but the vendor said that the software ran on RedHat, what's this CentOS thing? Better not take the risk."

RedHat is the household name that everybody knows, it probably makes sense to preserve it for the project that actually makes them money.

From the perspective of a 3rd party making software targeting RHEL it's simpler as well. I don't want to have to deal with clueless users installing an unsupported OS to run my software and then asking me for support for things like "our CD drive is not supported" or "how can we patch this security vulnerability". I've enough work supporting my software, I don't want to deal with the client's clueless sysadmin on top of it.