Actually, under the GPL, Red Hat is only obligated to make sources available to its customers. What they have done is make the sources available to everyone on the Internet for free [1]. So CentOS would have to pay for RHEL were it not for Red Hat's openness. Probably not a big deal. However, Red Hat is under no obligation to make its non-GPL packages (e.g., python, ruby, apache, postgresql, ssh, etc.) available to anyone in source form, including their customers. These, too, are available free of charge to the general public. Finally, Red Hat is under no obligation to make any of the source of their own internally-developed projects (e.g., package management, OS installer, and all of the other projects that differentiate the distribution from a software perspective) available under an open source license, but they do (admittedly, this was not always the case). Finally, Red Hat employs many developers who work full time on critical projects (kernel, gcc, gnome, etc.). They are pretty model open source citizens whose business model is not to use open source as a gateway to their own proprietary products like IBM and Oracle.

If they wanted to shut down CentOS, it would be very easy to stop distributing the source of their own projects and of permissive license packages. Hopefully sponsoring CentOS is not just a play to exert influence on the project and retard its progress, but I am willing to give Red Hat the benefit of the doubt here.

[1] http://ftp.redhat.com/redhat/linux/enterprise/6Server/en/os/...