Anyone can add a suffix to the public suffix list, if they give out subdomains to mutually unrelated parties. I added one. I don't think part of the mission of the PSL is to distinguish "domain" from "subdomain". It's certainly not meant to be a list of TLDs, just a list of suffixes below which the namespace is public.

The main distinction here is really just that keeping eu.org costs 20 bucks a year or whatever, and creating a subdomain costs them ~nothing, so it's not really remarkable that these are free, whereas, for example, a free .org would be remarkable.