While HN has a single stream of stories being voted on, the guidelines for story submissions limits the scope for what can be submitted. The people who read HN are pretty much all technically astute, so when they see an article behind a paywall they don't vote it up - heck, many probably just flag it on principle.

If the article is chock full of intrusive advertising, it gets flagged pretty fast. If it is an interesting article that gets to the front page I guarantee you that there will be vocal complaints about the advertising. But most of us here, I would hazard to guess, have installed uBlock Plus of some sort of ad blocker so we probably largely miss it. And the things we find interesting are usually from sources clueful enough not to be so stupid as to employ anti-adblockers.