Carpetsmoker
It is a complex answer.
Basically, Centos is Free for the download. Redhat does not provide binary images and basically justifies the purchase of RHEL with a bundled-in support contract. Since Redhat uses GNU Licensed code they honor the GPL by supplying the source code. At least most of the support code - some of the code required for the builds was left out this time.
More than you probably want to read about the delay here:
Centos Forum