For example of the second, there was a thread on /ck/ a few weeks back where some guy (probably a grad student; I'll never know) stumbled on a few $K of food lab equipment. Thread went on with a variety of experiments/projects which ranged from "reasonable" to "why would you even consider this" (some kind of flavored oil distillate from a happy meal, used to make ice cream).
If that was on HN, it would be someone's social-climbing portfolio blog, or I would have to wonder if it's astroturfing by some lab equipment company, or Mcdonalds. I'd question if this really was a curiosity-based endeavor, or if it was just someone trying to signal to potential employers "look at my Relevant Project!" or "look how quirky I am!" to friends.
But none of that was a concern; there is really no way for that individual to profit or benefit from this in any context. It's an anonymous forum, and there's strong social pressure to not subvert that (unless it is simply by virtue of posting similar content). It takes the game theory out of the equation; nobody is trying to sell me something.
It's not for everything or everyone, and there's definitely some effort in filtering out garbage posts. The same goes for HN, except the content to manually filter out is the sea of sometimes-veiled advertisements and self-promotion rather than plain-faced flamebait.