Just to add to the others CentOS was essentially bought by RedHat and there Core developer is now a RedHat employee. Magically there was a new major 7.0 version bump adopting SystemD as the one and only init system. A new flashy website and a refusal to allow any alternative init system into the official packages other then SystemD. Needles to say CentOS is no longer an independent alternative to RedHat Linux.
RedHat has to release there sources for others to use. Most of the code is under some form of GPL. CentOS was an independent volunteer driven effort that compiled and packaged that code into a new distro not affiliated with RedHat. That is why they had to remove RedHat branding and they could not market CentOS as RHEL etc.
