It's explained in the second link there as well as tons of comments I've posted about this (https://hn.algolia.com/?sort=byDate&dateRange=all&type=comme...).

We find the paywalls as annoying as anyone, but as long as there are publicly available workarounds, the articles remain accessible at the price of annoyance. This sucks, of course, but an HN without articles from any such publications would suck worse. Since the core idea of HN is to try to optimize for curiosity, we don't have much of a choice. Hopefully someday the publication problem will get a definitive solution and we won't have to deal with this crap anymore. In the meantime, publicly accessible workarounds are the best of a bad bunch.

(Re registration sites, it's the same principle: if there's a publicly available workaround, it's ok, if not it's not.)