Just because the architecture is free doesn't mean IBM won't make a shitload of money from the future of this processor. There are a lot of other ways than licensing. RHEL is free since forever and still it's the biggest income source for Red Hat afaik.
The subscription is not free of charge, the software is free of charge. If you buy a subscription and don't extend it or copy my filesystem you can use it without problems. You just won't receive official updates or official support.
However, it's important to note that although the sources for RHEL packages are available as open-source, that still doesn't make CentOS an exact replica of RHEL, and therefore doesn't make RHEL itself "free".
RHEL 8 was released almost 4 months ago and there's no CentOS 8 yet. Mainly because taking the "free" RHEL sources and rebuild them isn't a simple case of removing logos and other "non-free" material. The build process itself (the sequence in which packages must be built so that all of them build successfully and with equivalent functionality to RHEL) is a major piece of what makes RHEL, RHEL, and that isn't free (or even publicly available).
