Whether or not any "If you do stuff allowed by the GPL, we will stop doing business with you" clause in RedHats' support contract(s) is a restriction on the GPL is another question. No one has sued them yet.
Anyone distributing such patches is fully licensed to do so, however.
Another question is whether RedHats' distribution of collapsed sources (sources with all patches folded in, and very hard to separate back out) is in violation of the GPL, when the GPL clearly says that the sources that must be distributed are that form which is preferred for making modifications, and when RedHat clearly prefer working on the pristine-upstream+patches sources (as with any sane distributor). There's been long discussions on that subject on LWN before - best to read those.