Probably because a significant portion of HN readership already subscribes to the websites that commonly make it to the front page, and others know how to bypass the pay wall (i.e. https://archive.is/lk15F if you want to read this without a subscription).
I can't remember seeing any pay walled article on HN that wasn't bypassable using popular free services online. Usually, someone will also link one of the bypass websites pretty soon after a post starts gaining traction.
That the archive URL is some generally accepted thing here seems bizarre to me. I mean, taking the text and hosting it elsewhere—that’s just an unauthorized copy, just like if somebody had posted a torrent of a new popular movie so we could discuss it. Or links to cracked software (how else could those of us who don’t want to pay for it discuss it, right?)
I respond to the headline/other discussion in the thread if it is interesting. IMO, the publisher’s decision of how they want their work distributed and discussed should be respected. That includes not making unauthorized copies, and also the fact that that constrains conversation about their work.
I’m surprised that we seem to understand this for software but not written word. And we mostly act accordingly: lots of discussion of open source works, not so much on “paywalled” proprietary software.
This is in the FAQ at https://news.ycombinator.com/newsfaq.html and there's more explanation here:
https://hn.algolia.com/?dateRange=all&page=0&prefix=false&so...
https://news.ycombinator.com/item?id=10178989