It's an extremely common mistake to think that the pop psychology we pick up by sheer experience is a sufficient measure to understand what works. It's entirely too easy to forget that we, personally, are largely unaware of any proven methods for cultivating gardens. We're running off the anecdotal data that just happens to stick to our brains like burrs on a fur coat.
As others have duly stated, there are many people who are more than willing to pay for the privilege of trolling. It might not even start that way: I paid $40/mo to play a particular online game, and maintained my subscription as my play dwindled and my forum trolling increased. I eventually cancelled, but that was not an altruistic action: I simply needed to stop posting, and to stop paying for a game I didn't enjoy anymore. If someone has a similar experience with HN, you're relying on their self-control to end their own privileges.
Quite frankly, no one knows how to police a community perfectly. This is why religion was invented, after all, why politics exists. And see how that turned out, a few thousand years later. We're struggling damn hard, despite all the progress.
You never know what people will do. Every action, every ban, every harsh or kind word, every (pay)wall around the garden, every disincentive, carrot, stick, every reputation system, every voting system... you're going off your gut, at the end of the day, because there is no unified theory of sociology you can tap to actually justify it.