Or in there case, negative value. RH likes to have the world believe they are Linux. They don't want anything lessening their brand. Certification would make RH < UNIX I suspect which is not what they want. Certification has always been about the ISV's, and if they can convince the ISV to test on their implementation directly (and they have) they don't need it.

Well, in particular a lot of the kind of organisations RH sell to (banks) had experiences with Unix which were not that good: places I worked spent an enormous amount of money on very expensive machines with hardware-redundancy features which were at best marginally functional, and certainly not functional enough to rely on. These features compared very badly with the things that IBM Z-series machines could do (it might be that the IBM/AIX machines were better in this regard: I didn't deal with them very much).

Of course we'd argue that this is not the fault of Unix, and that's a different discussion. But the people who have spent 9-figure sums on all this marginally-functional tin that the Unix vendors foisted on them don't look at it that way: they just want something which is not Unix, and which runs on cheap tin. Linux is not Unix, and runs on cheap tin.