CentOS isn't providing any value-add. They simply strip branding and call it their own. That's not adhering to the spirit of the GPL, the idea that you take someone else's software, add value, and release that value into the wild. The only "value" they add is providing a way to circumvent Red Hat's distribution model for their binaries.

That's one of the fundamental freedoms of the GPL, yes. But it is hardly the only one. The GPL explicitly gives the recipients of the code the right to redistribute the code whether they "add value" or not. The mere act of redistributing code is arguably providing value; ask the folks at ITEC who got Ubuntu CDs that Leo burned. Again, Ubuntu has committed to that redistribution always being unencumbered, with Canonical support sold separately.

Precisely because RHEL is a package deal of freely-redistributable software and a support contract, its status is murkier. The deal under which RH got all that code to which they add value says that their customers have the right to make and use copies of the software. The RH distribution model comes rather close to the line of violating the spirit of the GPL. If CentOS etc. did not exist, some might think that it crossed the line. The existence of CentOS proves that RH is in compliance with their moral obligations to all those coders whose work they use.

The GPL is intended to ensure that if you enhance software (bug fixes, security fixes, feature enhancements, etc) you must provide the source for those when you give your program to the community. It's not intended to allow you to re-brand software and call it your own.