Where the GPL applies RedHat do however have to provide sources to any 3rd party though (given they distribute in binary only form in at least some situations). Which they do, hence CentOS.
Regarding the patches: Either the GPL applies to these patches, in which case RedHat must supply them to any 3rd party, and the RHEL contract makes no odds. Alternatively, the patches are not supplied because of GPL obligations, in which case this discussion above about the GPL and contract re the patches has been moot.
You can't get around the GPL "binary-distribution: supply preferred source to any 3rd party condition" via support contracts, just to be clear.