Pretty much this. Also US Govt and especially the millitary. When they buy "Linux" they buy RHEL. (Sometimes SUSE when they are working with Lockheed). Mostly because of support and RHEL5 and RHEL6 are certified with an alphabet soup of certifications (EAL4, FIPS-140-2, etc) and so that is what is used. Heck even RHEL7 doesn't have all those rubber stamp even though it is 2 years old almost now.