If it's in the kernel doesn't it have to be GPLv2? So only one person has to buy it, and they can share it with CentOS. Does CentOS have a special deal with RedHat that they won't do this?
That's a tricky question. As far I know Red Hat provides all the sources for their free software (that's the reason why CentOS can exist), but the binary things are a tricky question.
I think you could share your recipe (the source code), but retain the sauce (the configuration and compiler that produces the binary and the binary itself) and still comply GPL, it's a grey area.
Why CentOS wont share the Red Hat binaries? Well you won't want to bite the hand that feed you.