Content behind pay-walls should not be allowed for submission on HackerNews.
Not all paywalls are equally problematic, and most of this content is not truly inaccessible. There are standard workarounds that everybody already knows or can easily learn. It's not as if HN users are shy about pointing them out.
The status quo isn't great, but the brokenness of online content models is not something we can just fix by decree in our little corner of the web.