I honestly think people that complain about referral link are just inherently negative. A free link, that takes you to a promotion, that saves you money, and you want to point out that the guy who brought this to your attention, is gaining something positive, at no expense to you or anyone. You probably pirated the book too.
The problem is, once you start that on HN you'll have yet another reason for overposters to post every vaguely relevant special they see. I didn't mind that the poster did it (so bought via the referral) but it'd be sad if this caught on, and I wouldn't post a link with a referral myself. The benefits around here are karma, mutual respect and increased collective wisdom. Let's not find ways to dilute those further.
Thanks OP, great deal and I bought it.