RHEL source code is freely available under the GPL (GNU General Public License) for those who want to compile it themselves, but the actual finished product costs money. Yes, there is CentOS, a free-to-download clone of RHEL compiled from the source code by CentOS developers. But Red Hat charges a premium for RHEL because it's (theoretically) guaranteed to work—Red Hat and third-party software vendors make sure that applications running on RHEL are not broken when the operating system is updated.
"Red Hat Enterprise Linux, the bits, the ones and the zeros, are not free," Red Hat CEO Jim Whitehurst told Ars. "The source code is free and it's freely available. But we compile those bits and we make it enterprise-class."
Most importantly for major customers, Red Hat creates a long-term support edition every two years. It commits to support this for a full decade, to the point of taking critical fixes from the Linux community and back-porting them to the older versions.