I cannot speak for the original poster, but my observation in
some of these circles:
Lots of bundling the "others" into one bin/box. If a liberal expresses opposition for DACA, then it is automatically assumed that they are a right leaning person, and several related policy attributes are attached to the person.
Often, simply being undecided and questioning a particular stance leads to the same effect.
I've seen it on HN: A general lack of tolerance for certain viewpoints, with perfectly valid questions resulting in accusations. Unfortunately, more often than I would like. Easy examples would be a lot of threads that involve Trump.
BTW, case in point is this other comment:
https://news.ycombinator.com/item?id=16127579
Just because the two are well correlated in the US does not make them the same, and if you come from abroad, you will find many things taken as a given in the US (like the conflation of conservatism with religion, or right leaning tendencies with conservatism) that will confuse you. I suspect their experience in SV has been that when they try to bring it up, they get labeled.
I'm guessing Europe has a lot of radically right wing folks who like their national health care just fine. Imagine they move to a very Republican part of the US, and ask "what's wrong with universal healthcare?" and then are labeled as being liberal. Now do that in reverse in SV.
If you've lived a lot in the rest of the world, you will probably easily find lots of examples where liberals are not acting according to their principles, and the same for conservatives. So when someone tries to be what they think is a "real" liberal (which may imply adopting stances that the "other" side is in favor of), then they'll feel unwelcome.
Some day I'll probably write a more thought-out piece on this. The above is just rambling.