In stats class in uni we spent a good deal of time discussing the issues with different types of polling. Internet polling was "newish" at that point and so there were interesting debates going on with whether it'd suffer from the same effects as other voluntary polls (TL;DR it does).
When a poll is voluntarily responded to, even if the poll is eligible to the entire population you want to poll, you'll end up with some pretty heavy biases - first off the middle is always underrepresented - people who don't have strong feelings on a topic won't bother taking the time to reply and people who aren't familiar with the topic at all will generally (generally there are obviously people who reject this) decline to respond out of shame. So then you're left with the group of supporters and opposers - it turns out that, as a general rule, you should expect an across the board negative bias on any polls: if I asked "Is vanilla ice cream the best ice cream?" I'll end up getting a much stronger response from the people who disagree compared to those that affirm - when it comes to our attention things we don't like naturally draw more of our attention so that accounts for a decent part of the bias... Additionally the status quo tends to be underrepresented because people who are comfortable with the way things are are less likely to view broadcasting that belief as important... in general underdogs fight a lot harder.
So, back to your point... when discussing anything on the internet, regardless of the forum, expect the responses to be unrepresentative of the population in general and instead be composed of a higher proportion of dissenters than actually exists.
That all said, I do think HN has a notable left-lean. I think technology in general tends to underrepresent strongly religious individuals (there are exceptions: see larry wall) and the tech centers tend to congregate around educational centers which tend to be more liberal than the country as a whole.
Lastly, HN prides itself on being (or aspires to be) a relatively politics free zone - so a large number of people will downvote comments just because they lean in either political discussion because they don't want the conversation to turn into a miniature scale representation of modern US politics.
So yes, you get a strong reaction when advocating free markets just like other people get strong reactions when advocating socialist policies.