I don't use RHEL so I cannot be sure. However, the GPL permits "mere aggregation" of closed-source packages on the same media. Therefore, there may be packages (of necessity outside the kernel) that are Red Had proprietary closed source, which you don't get if you run Centos or any other Linux. Centos is now in bed with Red Hat and is basically a compiled version of all of RHEL that is GPL-licensed, for people who don't want to pay for support and any extra proprietary stuff (or as is often stated, simply to be sure of having someone to sue if it takes their business down!)
"Mere aggregation" also allows Red Hat to ship RHEL with copyrighted images (icons, screen backgrounds etc.) I don't suppose they'd give a damn if Centos accidentally shipped some of them, beyond asking politely for the next release to remove them. OTOH if they ever showed up in Oracle Linux, expect legal fireworks!