Because many of them are high-quality articles, and intellectual curiosity is what HN tries to optimize for.
Obviously the discussions on such articles aren't all crappy. Often they're good. That doesn't mean that off-topic generic tangents about paywalls aren't a problem. All generic tangents are a problem, and this was an increasingly common one.
It astonishes me how the people making objections in this discussion ignore that we're talking about articles that are possible for nearly everyone to read. That's what paywalls with workarounds means. It means readable with a bit of a nuisance.
There have been a few legitimate counterpoints—for example, if it's true that in some countries you can't google WSJ articles to read them, that's a problem. But mostly this argument has charged ahead as if we were talking about unreadable content, with lots of indignant points being made on that basis and little stopping to notice that it's false.