The key piece here is that Red Hat does follow an "upstream first" policy. Red Hat is the biggest corporate contributor to upstream Linux.
The upstream community, downstream distros, and Red Hat benefit from later versions of upstream Linux having the fixes Red Hat (and others) identified as necessary. The benefit comes in the form of a guarantee that the next time an upstream Linux kernel is selected as the starting point for RHEL++, Fedora++, SLES++, etc. the previously required fixes will all be in place (eliminating most concerns about regression: performance, feature, etc).
So it is in Red Hat's interest to continue its strong commitment to "upstream first". As a Red Hat kernel developer I can tell you that "upstream first" isn't just some mantra -- it governs my development workflow. If a change isn't accepted upstream it has an _extremely_ difficult time of getting into RHEL.