I may be wrong but they probably have a support contract with RHEL or similiar or they are using a sponsored distribution that caters to their specific needs such as Scientific Linux. I don't know if similiar projects/teams exist for BSD distros.
I doubt CERN has a formal run of the mill support contract with RH like other large organizations. Scientific Linux is developed (in-house) in partnership with Fermilab.[1]
For offical usage in the context of Oracle production databases and eventually other applications requiring commercial support there is a limited number of Red Hat Enterprise Linux 6 Server, Red Hat Enterprise Linux 5 Server licenses available at CERN. For all other tasks standard CERN Linux distribution: Scientific Linux CERN shall be used.
