I think you make a reasonable point, AND I note that there's a promotion at the top of the page.
"The following subscription-only content has been made available to you by an LWN subscriber. Thousands of subscribers depend on LWN for the best news from the Linux and free software communities. If you enjoy this article, please consider accepting the trial offer on the right. Thank you for visiting LWN.net!"
It's possible (this was posted on HN 5 hours ago) that text was added retrospectively, but working on the basis that it's boilerplate text then LWN seems to figure some subscriber links will be shared AND that's OK because it's an opportunity to promote subscription to others.
FWIW LWN posts subscriber links to HN themselves from time to time, as a promotion. I'm sure they could block access to subscriber links that saw "unusual" amount of traffic -- but I guess (whether or not this was posted by LWN or not) they usually assume that the more people that read one or two free links, the more people will subscribe (like libraries leads to increase in book sales etc).
