That they've read far too many threads of rabid unthinking "conservatives" and have now concluded that all conservatives are idiots. It's difficult to blame them to be honest.
Personally, I will always challenge idiots on my side, I don't often see the same on the other side, they tend to be much more unified and don't tolerate dissension.
That's commendable. I suggest that what you're seeing is also susceptible to perception bias. Regardless, HN is a place for civil and constructive discussion. However close-minded or obstinate we may believe others to be, it's counterproductive to express or engage on the assumption that they're unreasonable or irrational.
Then leave Reddit and Facebook for Reddit and Facebook. From my experience here, HN members really do value the community they foster here. Each of us is responsible for maintaining that, even when we — or others —sometimes slip.
Personally, I will always challenge idiots on my side, I don't often see the same on the other side, they tend to be much more unified and don't tolerate dissension.