I used to think this too, but after thinking about it a little more, I've come to the conclusion that the perception of bias towards a certain viewpoint in any thread is colored by the day of week (some old-timers frequent on weekends), the time of day (US lunch breaks and close of business), and the members who are drawn to the topics that you are interested in.
In other words, my perception of the "prevailing" bias tends to fluctuate with the time of day (I don't have a fixed schedule for visiting HN) or topics I currently have open.
Sometimes, out of curiosity, I click on a random thread (typically high comments/upvotes) with a title that I would ordinarily have not clicked on, just to get a sense of what the community is yapping about :).
And I am a european, though I don't see libertarian views beeing the majority at all ... rather the contrary ..
Transhumanist as in there will always be some guy in every biology/medicine thread to talk about how we will evolve as a species, or claim that immortality is achievable or the likes. This is especially prominent here (and was shocking to me at first), because these opinions are not so common otherwise (in society or online communities). I guess it's just the demographics and personal interests of people here that explain this phenomenon, but I was very surprised.
Can you explain more about this "irresponsible indifferent American fat and sick" thing you've encountered? As an American, I'm curious what has given you that impression specifically on HN.
But PT is not seen as viable, people enjoy living in suburbs and being reliant on private cars for anything, culturally less sensible to ecological matters overall compared to europeans (even though in Europe it scales, and I've felt people enclined to think ecologically most in Switzerland and Germany), mass consumption (people really love Amazon... I mean Europeans do too but Prime is less of a thing here).
Also in healthcare threads (which I browse a lot since that's my original field) I've often seen comments about massive weight loss, illnesses esp diet/digestive or diabetus related (like, in a scale I don't see as much amongst my regional communities). Also how people seem to discover once in a while what a healthy diet is (culturally less prevalent in the US than over here it seems).
This is probably especially shocking to me since I have above average interest in these topics, granted. But I'm still very shocked by the US culture which imo shows and prevails here in subtle things. But every now and then I'm like "ah, true, this is most probably an US user". At least it calms down the lust I could have after working there :)
Anyways as much as I like HN and its often insightful comments, this has proven me that I would have a hard time living/working in the US as we don't share the same values at all it seems. I kinda understand the valley's spirit better though, which is nice considering its huge impact on everyone's tech life.